The police have spoken out against introducing a 20mph limit on Islington’s main roads – effectively saying they will not be able to enforce it.

But Labour-run Islington Council has vowed to press ahead with the controversial plans – saying it is important to effect a “culture change” in the borough.

Last night, the council’s executive committee voted through a �423,000 plan to make the 25 main roads under its control – such as Hornsey Road, Blackstock Road and Caledonian Road –subject to the limit.

It will come into effect when the new signs are installed in March. The council has already made all its residential roads subject to the limit.

Only main roads run by Transport for London (TfL) such as Seven Sisters Road, Camden Road and the A1, will remain at 30mph.

The Metropolitan Police has objected and is being backed by Department for Transport guidelines. They state that 20mph limits should only be introduced in roads where the average speed is less than 25mph and where measures such as bumps or chicanes stop the traffic going any faster.

Traffic travels faster than that on 17 of the 25 main borough roads and the council is not planning to introduce any speed bumps or chicanes, relying instead on signs and road markings.

In a letter to the council, Pc Ralph Luker of the Metropolitan Police Traffic Management Unit, said: “Introducing an unrealistically low speed limit in these roads will create an enforcement problem where one does not currently exist.

“We will not routinely enforce 20mph speed limits and zones.

“Any pre-planned enforcement will have to be intelligence led and as a result of exceptional circumstances.

“If the council believes there is a real risk to vulnerable road users in those roads with high traffic speed then those problems might be better addressed with site-specific solutions.”

TfL has also expressed concerns – saying the number of accidents increased after a similar 20mph limit was introduced in Portsmouth.

But the decision has today been branded “ground-breaking” by pedestrian safety organisation Living Streets. Chief executive Tony Armstrong said: “This is a landmark decision which we wholeheartedly welcome and I commend Islington Council for recognising the groundswell of public support for 20mph speed limits where we work, live and play.

“This is the beginning of a culture change where speeding is socially unacceptable. Just recently there have been two needless deaths as the result of road traffic collisions in London. The introduction of 20mph speed limits across the capital is probably the most effective measure to avoid such tragedies in the future.”

Caroline Russell, chair of the Islington branch of Living Streets, who led the local campaign, said: “We have worked to build support for 20mph across communities in Islington over many years. Last night’s visionary decision by Islington Council is a game changer for road safety and making London a more liveable city.

“We don’t want to stop there. We want 20mph limits on Transport for London roads like Holloway Road. It may be the A1 going all the way to Edinburgh, but as it passes through Islington it is a residential and shopping street a place where people live, work and shop that desperately needs slower traffic.”