Sir Keir had already faced backlash for the deal after it emerged that it would likely mean only around 50 migrants would be returned to France each week – a small fraction of the numbers crossing, which have reached 21,117 so far this year.

Staggeringly, Thursday’s arrival numbers would take three months to return to France.

Mr Macron, who blamed Brexit for making it harder to tackle illegal migration, said the measure would have a “deterrent effect” beyond the numbers actually returned.

Sir Keir said the “one-in, one-out” returns scheme will begin within weeks and help “finally turn the tables” on the migrant crisis in the English Channel.

He labelled it “groundbreaking” and warned those crossing that it meant “you will end up where you started”.

Under the plan, migrants will be sent back to France, with an asylum seeker being sent to the UK in exchange.

Reform UK leader Nigel Farage said the Prime Minister had “bowed down” to Mr Macron.

“We have acted today as an EU member and bowed down to an arrogant French president,” he said.

Writing exclusively in the Daily Express, Shadow Home Secretary Chris Philp said the PM had “caved in” over the deal.

“Labour’s limp deal will only return one in every 17 illegal immigrants arriving,” he said.

“Allowing 94% of illegal immigrants to stay will make no difference whatsoever and have no deterrent effect.

“This is the latest catastrophic example that when Labour negotiates, the UK loses. Starmer has caved in again.”

The Prime Minister set out the plan at the conclusion of Mr Macron’s three-day state visit to the UK.

At a joint press conference at Northwood Headquarters in London, he said: “There is no silver bullet here, but with a united effort, new tactics and a new level of intent, we can finally turn the tables.”

Under the “groundbreaking” pilot scheme, “for the very first time, migrants arriving via small boat will be detained and returned to France in short order”.

Sir Keir said it “will show others trying to make the same journey that it will be in vain”.

The Prime Minister promised that the jobs migrants have been promised in the UK “will no longer exist because of the nationwide crackdown we’re delivering on illegal working, which is on a completely unprecedented scale”.

His comments follow Mr Macron’s warnings about the “pull factors” luring people into travelling through Europe to reach the northern French coast in the hope of reaching the UK.

And in a dig at Mr Farage, who was out in the Channel on Thursday, Sir Keir added: “It is of some significance, I think, that while we have been working hard to get a returns agreement, others have simply been taking pictures of the problem.”

Mr Macron also reopened old Brexit wounds by blaming the withdrawal from the EU for the migration crisis.

He told reporters: “Since Brexit, and I’m saying all this quite honestly, I know it’s not your case, Prime Minister, but many people in your country explained that Brexit would make it possible to fight more effectively against illegal immigration.

“But it’s in fact since Brexit [that] the UK has no migratory agreement with the EU.”

He added: “It creates an incentive to make the crossing, the precise opposite of what Brexit had promised.”

Referring to “renewed trust” between the UK and Europe after Brexit, he later said the British people were “sold a lie … which is that the problem was Europe, but the problem has become Brexit.

“With your Government, we’re pragmatic, and for the first time in nine years, we’re providing a response.”

Thursday’s migrant crossings were the first since last Friday, when French police were seen wading ankle-deep into the water and puncturing a dinghy with knives.

More than 44,000 migrants have successfully made the crossing since Sir Keir became Prime Minister a year ago.