Thousands of motorists may have been stung with unlawful parking fines, it has emerged.

Drivers issued with penalties for leaving their cars in suspended parking bays may be able to fight for a refund after it was revealed that Croydon Council has no government authorisation for the signs it uses to warn them.

Department for Transport (DfT) rules state that councils must apply for approval of signs to advertise suspended bays - sites where parking is normally allowed but has been temporarily banned.

But Croydon is one of 16 London boroughs in which the council has not applied for such approval.

The council fined 526 motorists for parking in suspended bays between January 2009 and June 2011. It did not have figures dating back earlier.

A BBC investigation found that at least 350,000 tickets have been issued at bays with unauthorised signs across London.

Campaigners say that motorists have a right to reclaim fines issued unlawfully.

Paul Pearson, founder of pressure group Penalty Charge Notice, said: "When a motorist makes a mistake they receive a hefty fine from the council so it seems only fair that when a council makes a mistake they refund any money unlawfully taken.

"The fact that they have paid should be of little consequence as they did so under the assumption that the restriction was lawfully signposted."

In 2010 a ticket issued to a motorist in Camden who parked in a suspended bay was ruled unlawful because the council had not applied for authorisation.

But Croydon Council insists the fines are legal because of a second test case in 2011 in which the Court of Appeal ruled that failure to comply with DfT did not invalidate it signs.

A spokesman said: "The case in 2010 set no legal precedent and there have since been many others where fines have been upheld.

"The 2011 Court of Appeal ruling confirms that as long as signage is clear then there is no basis for a successful challenge."

The council's position is supported by lobby group London Councils.