Earlier we heard from TfL's Nick Dent, who says that negotiations have primarily centred on "operational details" rather than on issues of safety.

But the RMT's statement following yesterday's failed talks stresses safety concerns among its members is still their core complaint.

"TfL have failed to provide assurances on our members deeply held concerns around fatigue, reduced flexibility, shift lengths and the impact these proposals could have in a safety-critical role like tube driving," the union's representative says.

The RMT isn't just at odds with TfL over accepting the voluntary four-day week, but also its fellow drivers' union, Aslef.

That union, which represents a bit more than half of Tube drivers, previously said it was "surprised that the RMT has decided to take this action".

Transport for London's director of customer operations, Nick Dent, says it is "very disappointing" that the RMT has gone ahead with today's strike action.

He tells BBC Radio London that he understands the "frustration" and "disappointment" passengers and businesses are feeling because of the disruption.

Dent rejects the RMT's claims the dispute is about safety, saying: "Safety is our absolute priority."

He says the proposed changes to driver working arrangements are supported by Aslef, the other main drivers' union, and are consistent with arrangements used elsewhere on the national rail network.

Dent says discussions with the RMT have focused on operational details rather than concerns about safety or fatigue.

TfL says the changes are voluntary and drivers who want to remain on their existing agreements can do so.

Dent acknowledges there are still "lots of operational details" to work through, but urges the union to continue discussions rather than take industrial action.

"There's no need to take strike action in the meantime," he says. "It won't help us sort those details any faster."

Sameena Misbahuddin
Reporting from Stratford station

I began my morning at Stratford station where most trains are running normally, though the Central line is stopping once it gets to Liverpool Street.

There are long gaps between trains and people are turning to the Elizabeth line, which is much busier than usual as it is the only option east to west through central London.

The Jubilee line is running fine at the moment but staff say there had been a delay in the service starting in the morning.

With possible problems today, staff at the station have written up a helpful list of alternative routes.

It's a mixed story across the Tube network this morning, with disruption ranging from minor to severe across the various lines in London.

Here's the latest update across each line:

• Bakerloo line - The line is part suspended between Queen's Park and Harrow & Wealdstone and there are severe delays on the rest of the line
• Central line - There are minor delays on the entire line, which is part suspended between Liverpool Street and White City
• Circle line - The entire line is suspended
• District line - There are severe delays across the entire line, with services operating approximately every 10 miuntes, according to TfL
• Hammersmith & City - Services are also operating every 10 minutes on this line, with severe delays across the entire line
• Metropolitan line - There are minor delays on the entire line, which is part suspended between Baker Street and Aldgate
• Northern line - Services are operating approximately every six minutes, with minor delays across the entire line
• Piccadilly line - There are severe delays between Cockfosters and Arnos Grove. The line is also part suspended between Arnos Grove and Uxbridge, Heathrow Terminal 5, and Heathrow Terminal 4
• Victoria line - There are severe delays across the entire line, with services operation every four minutes
• Waterloo & City - There is no service across the entire line
• Jubilee line - This line is operating a good service with no delays
• London Overground - There is good service across all lines

Gem O'Reilly
Reporting from Victoria station

There's a feeling of agitation this morning down here at Victoria Station. A lot of commuters have been telling me about their frustration in being late for flights, work and appointments.

One woman from Brixton says she had to take two buses before she got to Victoria, and I caught her as she was boarding a third bus.

“I’m late for work and work in a school. I should be there by now," she says.

“I’m really tired. Also coming back home it’ll take me hours to get back and as a mum it’s not good.”

Other people tell me they were expecting certain Tube lines to be working and they were entirely suspended, so they had to completely re-route.

One man says it’s added an hour on to his journey and that it’s very frustrating, with the queues being the most stressful bit.

The bus and taxi queues at Victoria have been filling up since I’ve got here at 06:00 and now people are becoming more impatient.

Harry Low
Reporting from King's Cross

One of the lifts at King's Cross St Pancras is closed.

The electronic display says it is under staff control - but there is nobody at street level to approach, restricting step-free access to some London Underground passengers this morning.

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Tom Edwards
Transport Correspondent, BBC London

At 07:00 BST it was business as usual at Hanwell Elizabeth line station.

Lots of commuters have switched to Elizabeth line services as its drivers aren’t on strike.

It makes a huge difference travelling across the city and that is why Transport for London thinks it can suspend the central parts of the Piccadilly line and the Central lines.

This allows it to concentrate the drivers it does have working today on other parts of the network.

Rob Oxley
BBC Radio London

The Northern line southbound from Brent Cross is very quiet this morning and running with trains every few minutes.

But it’s a different story northbound with long gaps between the trains.

Harry Low
Reporting from King's Cross

Nurse Susan Ajayi is worried that her journey will take as long as during April’s Tube strikes when it took her four hours to get to work having left home at 05:45.

“It’s going to be worse because of the rain - it’s the wrong day to have a Tube strike I’m afraid,” the 60-year-old tells me.

“I’ve got patients to meet. I’m dreading taking the bus in this weather.”

Image source, Reuters

The RMT union has rejected an offer for drivers to voluntarily work a four-day week.

Under the proposals, most drivers would see their working week reduced from 36 hours to 35 hours. Their contractual hours would remain the same because paid meal breaks would be introduced.

This effectively means drivers would work longer days, but fewer days each week and fewer overall hours each week.

The RMT union said the working day would be too long under the plans, risking driver fatigue and compromising safety. The union instead wants a 32-hour week over four days, meaning drivers would work three fewer hours each week for the same salary.

TfL said drivers could remain on a five-day week if they wanted to, and added that the change was voluntary.

Harry Low
BBC London

At King’s Cross St Pancras this morning, the late start to the few services which are running has caught plenty of people out.

Lots of scratched heads from commuters and furious scrolling of phones before the gates to the Northern and Victoria line eventually opened - with severe delays.

The Piccadilly and Circle lines are not operating from here, however.

Briohny Williams
BBC London

Another person I spoke to was a coffee shop worker who said that people who worked nearby were often "prepared to work from home regardless of whether the Tube strike goes ahead or not".

"Our business is impacted as there are less people coming into the shop," she said.

She added that on strike days, it "was a struggle" to get into the coffee shop to start her shift.

"We open early regardless of the Tube strikes," she said.

Briohny Williams
BBC London

On Monday I spoke to London business owners about how another Tube walkout would affect them.

Delicatessen owner Simone Di Iulio told me the Tube strikes were "chaotic" and that during the last strike, he had to pay for a taxi to commute to work.

He said strikes made a big difference to footfall in the area, adding: "I know a lot of office people can work from home, and if that's the case, there's going to be no one in London."

Di Iulio said that with rain forecast, he anticipated fewer people cycling to work, and more people staying home.

Today's Tube strike begun at 00:01 and will run until 23:59 BST. Another 24-hour strike will begin at 00:01 on Thursday.

The action is different to the strikes in April, which saw 24-hour walkouts begin from midday, creating an impact across four days within the week.

The difference means the strike action will largely impact two days, instead of four.

As not all train drivers are striking, TfL says it hopes to run around 50% of services during the day.

Currently a number of lines are suspended or part suspended, although this is likely to change over time.

You can get live updates about what trains are running on TfL's website., external

Ahead of the strikes, Transport for London (TfL) said it hoped to run at least half of its trains, with disruption expected on the following routes:

• No service is expected on the Circle and Piccadilly lines
• No trains on Metropolitan line between Baker Street and Aldgate
• No Central line service between White City and Liverpool Street

TfL said it expected to run services on these Tube lines:

• Bakerloo, District, Hammersmith and City, Jubilee, Northern, Victoria and Waterloo and City

No service was expected on any line before 06:30 BST and Tube trains are also expected to finish earlier running than usual this evening.

Good morning and welcome to our coverage as another round of strikes have begun on the London Underground.

Members of the Rail, Maritime and Transport (RMT) union walked out from 00:01 BST on Tuesday and will continue striking until 23:59.

Another strike is planned for Thursday 4 June from 00:01 to 23:59 BST.