Norwich City Council thought it had driven out non-resident parkers from the streets around the university by introducing a controlled parking zone.

But the Department of Transport has told the council the parking ban cannot be enforced because they ordered yellow signs instead of regulation white.

The mistake could cost taxpayers thousands of pounds to put right.

It will also prolong the frustration of people living near the university who have already paid £16 for a parking disc allowing them to leave their cars outside their homes.

'Sorry' for mistake

A council spokeswoman said 18 of the wrong coloured signs had been erected and would be replaced with the correctly coloured signs as soon as possible.

"We have made a mistake and are very sorry. The incorrect colour signs were ordered. They should have been white, but are yellow instead.

"Replacement signs have been re-ordered and should be up in the next few weeks. Unfortunately, under traffic regulations, signs must be correct to be enforceable - this includes colour.