Keir Starmer has said he is not in favour of changing the law to decriminalise drugs, following reports that London Mayor Sadiq Khan is planning to end the prosecution of young people caught with cannabis.

The Labour leader and MP for Holborn and St Pancras said he has not seen the details of the reported proposals, which three London boroughs – Lewisham, Bexley and Greenwich – are apparently set to pilot.

Under the scheme, which is set to launch later this year, police officers would be told not to arrest young people caught with cannabis, ketamine or speed, and under-25s would instead be offered speeding course-style classes or counselling.

The office of the Labour mayor of London said it is misleading to suggest that Mr Khan is moving to decriminalise the three drugs.

It said plans for a trial are still in development and would only apply to “18 to 24-year-olds found in possession of a small amount of cannabis” and “would not apply to any other drug”.

The pilot would be based on a successful Thames Valley model which won a national award last year, the mayor’s office said.

The pilot has not yet had approval from the Mayor’s Office for Policing and Crime (Mopac), Mr Khan’s office said, and the mayor does not have the power to decriminalise cannabis.

Asked about the proposals, Sir Keir said: “On the drugs legislation, I’ve said a number of times and I will say again: I’m not in favour of us changing the law or decriminalisation. I’m very clear about that.

“I haven’t seen the detail of the proposals that you’ve reported on. As I understand it they are early measures, they are some sort of pilot.

“Obviously we’ll look at those, but I’m very clear that we’re not in favour of changing the drugs laws.”

A spokesman for the Mayor of London said: “The idea of the scheme, which is already used by other police forces across the country, would be to divert young people who are found with a small amount of cannabis away from the criminal justice system and instead provide help and support."