As Rishi Sunak prepared to meet Emmanuel Macron in March 2023, his migration minister wrote to him to propose some radical proposals that might persuade France to crack down on small boat crossings.

Robert Jenrick, now the shadow justice secretary, wrote: “We should be willing to make highly meaningful concessions such as offering to take one asylum seeker that has been successfully granted asylum in the French system in return for one illegal migrant that we return to them.

“Or indeed more than one. It is highly probably that the certainty of return to France would quickly break the business model of the smugglers.”

On Thursday, his advice was heeded, but by Keir Starmer rather than the prime minister Jenrick served. By agreeing a migrant returns deal with France – albeit a limited pilot on an uncertain timescale – this prime minister has achieved something successive Conservative ministers attempted but failed.

Starmer said on Thursday: “It’s no wonder that for years, previous governments have been trying to secure a deal, a returns agreement like this, because all governments have realised just how important it is.”

Under the deal announced by both leaders from the military headquarters in Northwood, north-west London, British officers will detain a reported 50 people a week and send them back to parts of France away from the northern coast.

Their biometric data will be taken at that point and should they return again by small boat to the UK, they will be on record and will be returned again, officials said.

Though officials said it was possible that human rights appeals could be made, the Home Office said it was confident it would win any challenge, given the safety of France.

Those in France wanting to take advantage of the new safe and legal route the government has just opened up can do so by applying via an online platform, which officials say has already been developed and tested by the Home Office.

It will involve a visa application process with security checks before being admitted to the UK, where people will be allowed to claim asylum legally.

Many questions remain about the scheme, however, not least when it will start.

Macron and Starmer said they expected it to begin “within weeks”, but the French president added that he wanted to secure the agreement of other European nations first. That could prove tricky given concerns in Mediterranean countries that people being returned to France could then make their way back south.

Nor is the scale certain. Neither leader denied reports that the pilot would aim to send back 50 people a week – just one in 17 of the total number who make the crossing on average. British advisers say they hope it will increase in scope if it is shown to help bring down numbers.

Officials are also unable to say how they will choose which asylum seekers they will return to France or allow to enter Britain. UK officials say they will prioritise nationalities that are most vulnerable to people-smuggling and those most likely to have successful claims to asylum and links to the UK.

Children are not detained when they cross on small boats, so officials said they would not be part of the process.

The two parties say Britain will not make any extra payments to support French policing as a direct result of this deal. But Paris has been asking for increased funding for equipment such as drones and extra officers, and Macron bristled at suggestions that the money had not been used well.

“I wouldn’t want there to be an idea that we are not doing our job or wasting UK money,” he said. “We have cut off the land route, the rail route, and sharply reduced that of the small boats. These smugglers are adapting – we must adapt.”

Even with these uncertainties, Labour advisers are delighted they have struck a deal where their predecessors failed.

They say a combination of factors helped get it over the line in recent days, including British commitments to do more to tackle illegal working in the UK and the trust that has been fostered between the two men.

The Conservatives described it a “migrant surrender deal”, with the shadow home secretary, Chris Philp, calling it a “green light to people-smugglers”.

Jenrick, a politician normally expert at capturing media attention, was uncharacteristically quiet.