Sir Keir Starmer is facing fresh embarrassment over his pledge to 'smash the gangs', after French police refused to stop small–boat migrants from crossing the Channel by incapacitating their vessels.

Unions representing the officers have blocked plans for police to stop the dinghies by using tactics such as jamming the propellers, on the grounds that they could face prosecution if any migrants died as a result.

This comes despite year–long negotiations between Sir Keir and French president Emmanuel Macron over the interceptions – and the payment of £500 million by the Government to France. In return for the money, which covers a three–year period, Paris is meant to act to prevent migrants from reaching British shores.

In a letter written by Sir Keir to Mr Macron last month, the Prime Minister said: 'It is essential that we deploy these tactics this month,' adding: 'We do not have an effective deterrent in the Channel.' 

But now Alliance, the largest police union in France, has said it is too dangerous for its members to carry out arrests at sea while trying to force a boat to change course.

A source at the French interior ministry told The Mail on Sunday: 'The police want guarantees that there will be no prosecutions in case of death or injury, but prosecutors say that is impossible. Meetings between lawyers and police chiefs have taken place over the past month, but the judicial authorities remain firm – there will be no waiving of criminal liability if things go wrong.

'Any interceptions at sea are extremely dangerous, and the risk has to be accepted by those involved in policing immigrant boats.'

Last night the Home Office tried to put a brave face on the latest setback, with a source insisting the Government was still 'committed to working in partnership' with the French to 'drive down small–boat crossings and break the model of the criminal smuggling gangs'.

Unions representing the officers have blocked plans for French police to stop the dinghies by using tactics such as jamming the propellers

Sir Keir Starmer is facing fresh embarrassment over his pledge to 'smash the gangs', after French police refused to stop small–boat migrants from crossing the Channel by incapacitating their vessels (Sir Keir,left, pictured with French President Emmanuel Macron, right, earlier this month) 

But the Conservatives said the revelation demonstrated that it was a serious mistake for Labour to cancel Rishi Sunak's plan to deter migrants by threatening to deport them to Rwanda.

Shadow Home Secretary Chris Philp said: 'Keir Starmer promised interceptions at sea near the French coast, and that's proved to be another gimmick that won't happen – just like his claim to smash the gangs and return many illegal immigrants.