By Tim Bowler Business reporter, BBC World Service

Dockyard's troubled history

Britain's Ministry of Defence is trying to block a share flotation by one of America's biggest defence companies.

Halliburton had been planning a stock market launch for its subsidiary KBR, a firm that has been mired in controversy over its business operations in Iraq.

But now Britain has asked the firm to delay the launch - or risk losing its contract to operate Devonport dockyard, western Europe's largest naval port.

The sell-off was planned to generate up to $470m (£248m).

Now the key strategic move risks being scuppered by Britain's Ministry of Defence, which is not happy about the potential implications of the sell-off for the dockyard.

Warning

The dockyard in the south-west of England is crucial to Britain's nuclear deterrent.

Both Halliburton and KBR have been involved in reconstruction in Iraq

It is the only place which is equipped to refit and refuel the country's nuclear submarine fleet.

It is currently owned by Devonport Management Limited, in which KBR has a controlling 51% stake.

The British Government is concerned whether KBR, once it is spun off from its parent company, will have the cash to successfully run Devonport - given that it will no longer be able to rely on support from Halliburton.

Britain says it wants to delay the sell-off while it takes a closer look at KBR's finances.

And the British Government has given the Americans a blunt warning.

If at any time it feels the UK's essential security interests are threatened, then the Ministry of Defence has the right to assume control of the dockyard.