The UK and France have been forced to rewrite the “one in, one out” deal because of concerns over the numbers of people re-entering the UK after being removed to the continent.

The original treaty said people arriving in small boats could be returned to France. But people smugglers have used lorries to bring people who had been deported to France under the deal back to the UK.

Shabana Mahmood, the UK home secretary, has agreed with her French counterpart to amend the treaty so that previously deported “one in, one out” migrants who return to Britain by lorry can be sent back to France again.

To close the loophole, the Home Office created a new classification of claimant, a “returnee case”, according to documents first disclosed by the Times.

The UK has removed to France 921 migrants who arrived on small boats since the treaty came into effect on 6 August last year, which represents 3.5% of all such arrivals in the same period.

The UK has accepted 896 asylum seekers from France in that time, under the reciprocal deal.

The treaty stipulated that Britain could send illegal Channel migrants back to France in return for taking the same number of asylum seekers.

At least four people who had been deported under the scheme travelled back to the UK by lorry over a two-week period in March and this followed at least two in the autumn.

Mahmood and Laurent Nuñez, the French interior minister, have agreed to apply the treaty to any returning migrant entering the UK illegally, regardless of their method.

In a letter to Nuñez, Mahmood said: “Following our recent meeting which allowed us to observe the quality of the cooperation established under the agreement … I wish to propose an addition to the objectives of the agreement, explicitly adding the objective of deterring clandestine returns to the UK by individuals previously transferred to France under the agreement.”

People removed to France under the agreement who then travelled back to the UK previously told the Guardian: “The [people] smugglers know where the shelter is in Paris where people sleep in the first few days after being returned to France.

“They caught me near the shelter and sent me back to UK by force in a lorry. The smugglers have guns, they control everything, we have to try to stay alive.”

The UK prime minister, Keir Starmer, and the French president, Emmanuel Macron, signed a deal they hailed as “groundbreaking” last July, known as “one in, one out”.

Under the terms of the deal, one asylum seeker who arrives in the UK in a small boat is forcibly returned to France, in exchange for another in France who has not tried to cross the Channel being brought to the UK legally.

The two leaders agreed that initially the scheme would be a pilot, which was due to end on 11 June. But the two countries have now agreed to extend the scheme until 1 October.

The numbers crossing the Channel so far this year have reduced by approximately a third compared with the same period last year, although this is thought to be partly due to the weather.

In April, the UK and France confirmed a new £662m deal to stop migrants from crossing the Channel.

A Home Office spokesperson said: “Under our returns agreement with France, we have already removed more than 900 illegal migrants from British soil.

“This contributes to the nearly 70,000 illegal migrants who have been returned from July 2024 to March 31, 2026, up 41% on the 21 months prior.”