Islington has been blocked by Haringey from applying its flagship 20mph scheme to every main road – just weeks after the boroughs trumpeted a new agreement to work closer together.

Stroud Green Road in Finsbury Park will be the one principal road left out, because it forms the boundary between the two boroughs and Haringey has refused to allow it.

The development is an embarrassing blow for inter-borough teamwork as it comes so soon after the councils penned their Finsbury Park Accord.

Cllr Terry Stacy, leader of the Lib Dem opposition at Islington Council, said: “Just weeks ago, the Labour leaders of Islington, Haringey and Hackney promised to work together to improve life for residents living along the borough boundaries. This decision by Haringey makes a mockery of that.

“We are now faced with a situation where one of Islington’s busiest roads will remain one of the most dangerous.”

The accord was supposed to usher in a new era of cooperation and Stroud Green Road was cited as one of the main streets that would benefit.

Yet drivers travelling along Stroud Green Road and Crouch Hill – which it turns into – now face a confusing situation where the speed limit will chop and change between 20mph and 30mph sections.

Bizarrely, the other main boundary road, Hornsey Lane, is already 20mph – and was Haringey’s decision.

Road safety campaigner Caroline Russell, chairman of Islington Living Streets, said: “It’s quite hard to make sense of it and there needs to be an outcry. It’s just incomprehensible.”

Both Camden and Hackney councils have agreed to the 20mph limit on all the other fringe roads.

Cllr Nilgun Canver, Haringey Council’s cabinet member for environment, said: “We have not ruled out the proposal entirely, but we do not support the introduction of 20mph speed limits without there being sufficient traffic calming measures in place. As the number of accidents is relatively low along this road we do not see the benefit of this proposal.”

Only major thoroughfares like Holloway Road, which are run by Transport for London and are completely out of the councils’ hands, will stay at 30mph