Priti Patel is set to anger France by sanctioning plans to forcibly redirect migrant boats back across the Channel despite warnings the controversial tactic would deepen a rift in relations.
The home secretary has reportedly ordered officials to rewrite maritime laws to allow Border Force to turn boats around, forcing them to be dealt with by French authorities.
It comes following a G7 interior minister's meeting on Wednesday, during which Ms Patel told her French counterpart that the British public "expect to see results" from French efforts to prevent ongoing migrant crossings.
Several newspapers reported that members of Border Force are being given special training to handle migrant boats, but would only deploy the "pushback" tactics when deemed practical and safe to do so.
Campaigners have previously described the plans, first outlined in July, as both illegal and “morally wrong”.
Reports suggested such operations were likely to be restricted to sturdier, bigger migrant boats and only used in "very limited circumstances".
Ms Patel and the French interior minister, Gerald Darmanin, held discussions on crossings at Lancaster House in London, in the wake of hundreds of migrants being brought ashore in Kent over the past few days.
UK government sources told the PA news agency the pair had a “constructive” meeting in which Ms Patel made clear tackling the number of people making their way from France to the UK on small boats was her “number one priority”.
But the French government warned ahead of the meeting that the newly reported turnaround tactics would have "a negative impact on our co-operation".
In a letter seen by The Independent, Mr Darmanin told Ms Patel: “The French position on intervention at sea remains unchanged. Safeguarding human lives at sea takes priority over considerations of nationality, status and migratory policy, out of strict respect for the international maritime law governing search and rescue at sea.
“With regard to traffic and conditions for crossing the Channel, France has no other solution than to intervene most often on the basis of the provisions in international law governing search and rescue at sea.
“The use of maritime refoulement [pushbacks] to French territorial waters would risk having a negative impact on our cooperation.”
The French interior minister also rejected a UK request to set up a joint command centre in northern France, with police and border force officers from both countries patrolling the coastline and the Channel.
It comes just days after Ms Patel is said to have told MPs she is prepared to withhold millions of pounds of cash promised to France to help step up patrols unless an improvement in the number of migrants intercepted by French authorities is seen.
A UK government source said: "The home secretary was clear with the French interior minister that the British public expect to see results."
Earlier on Wednesday, prime minister Boris Johnson said efforts to stop the crossings depended "to a large extent" on the ability of the French authorities.
According to the Home Office, 785 migrants arrived in the UK on Monday after making the journey from France in small boats, with several young children and a baby among them.
This is the second highest daily total of the year, following the single-day record of 828 people set last month.
The crossings continued on Tuesday and Wednesday during the better weather conditions, with boats arriving at Dover and others being towed on to beaches along the south coast.
Earlier this year, the UK and France announced an agreement to more than double the number of police patrolling French beaches.
It was the second pledge of its kind in a year, in a bid to stop small boats from leaving France.
As part of the deal, the government pledged to give France £54 million to support its efforts to stop small boat crossings.
Charities urged the Home Office to take a "more humane and responsible approach" towards asylum seekers and said humanitarian visas were needed to help "prevent the chaos of the Channel crossings".
Pierre-Henri Dumont, who represents Calais in the French National Assembly, told BBC Radio 4's Today programme: "Nothing can stop them.
"The fact is, we've got 300 to 400 kilometres of shore to monitor every day and every night and it's quite impossible to have police officers every 100 metres because of the length of the shore."
At least 13,000 people have now made the crossing to the UK in 2021, according to data compiled by the PA news agency.
According to the UK government's clandestine channel threat commander Dan O'Mahoney, efforts so far have prevented more than 10,000 migrant attempts, led to almost 300 arrests and secured 65 convictions.
Illegal migration is anticipated to be the subject of focus for the final day of the G7 meeting on Thursday.