Hundreds of drivers are said to have been wrongly fined after a fake 50mph sign was placed in a 40mph speed zone – but the Metropolitan Police has said the fines are still “within the law”.

The sign was erected on the A20 in Sidcup, southeast London.

Transport for London reduced the speed limit on the road last year “in response to a number of incidents caused by ongoing flooding issues”.

The Met said it has now launched an investigation into the incident as an attempt to “pervert the court of justice”.

But the force said it is satisfied the fines issued are “within the law” as there were several other 40mph speed signs in the area.

Dominic Smith, director at Patterson Law, a firm that specialises in motoring offences, said he has never “seen anything quite like this, on this magnitude”.

He added: “We’ve been contacted in the last week by about 400 or 500 individuals, of which about 100 to 150 are at risk of losing their licences because of this.

“Usually when a new speed camera goes up, we can tell here because we get maybe about two or three enquiries a day for a couple of days – 40 to 50 a day we’re getting at the moment. I’ve never seen anything like it.”

A Transport for London (TfL) spokesperson said: “For a short period, an incorrect 50mph sign was illegally placed at one location on the road by somebody who had not been authorised by TfL to place it here, putting people travelling at risk.

“This was immediately replaced with the correct 40mph signage. All other signage is correct and compliant with the regulations.”

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The spokesperson added: “Safety is our number one priority and we temporarily introduced a 40mph speed limit on the A20 Sidcup Road in response to a number of incidents caused by ongoing flooding issues, which could have posed a risk to life.

“It is important that people follow the new speed limit to ensure that everyone can travel safely along this road and safety camera enforcement is a vital part of this.

“We want to ensure that all drivers are treated fairly and new, regular speed limit signage, compliant with all traffic sign regulations, was installed to ensure that everyone driving here is aware of the new limit.”