Boats carrying migrants across the English Channel could be turned back from the UK, if personally approved by Home Secretary Priti Patel.
The government has authorised Border Force officials to use the new tactic – but only in limited circumstances.
However France is strongly against the plan, saying it breaks maritime law and accusing the UK of blackmail.
But Boris Johnson’s spokesman said any plans would be “safe and legal” and would comply with the law.
Rising numbers of migrants have been crossing the English Channel in recent months – and so far this week more than 1,500 people have crossed by boat.
The Channel is one of the most dangerous and busiest shipping lanes in the world. Many migrants come from some of the poorest and most chaotic parts of the world, and many ask to claim asylum once they are picked up by the UK authorities.
Government sources have confirmed to the BBC that a Border Force team has been training for months to begin the operation.
It is understood that the final training may take place within days, subject to the weather – meaning the tactic would then be ready to be used whenever is it practical and safe to do so.
But France says the move flouts international maritime law, which says people at risk of losing their lives at sea must be rescued.
France’s interior minister Gérald Darmanin – who met Ms Patel on Wednesday for talks on the migrant crisis – also accused the UK of financial blackmail.
He was referring to a deal the UK and France struck over money earlier this year, when the UK promised to pay France £54.2m for extra action such as doubling the number of coast patrols. Since then, Ms Patel has warned that Britain could withhold the money, unless more boats are intercepted.
“UK’s commitment must be kept,” said Mr Darmanin. “I clearly said it to my counterpart Priti Patel. The friendship between our two countries deserves better than posturing which undermines the cooperation between our ministries”.
The government’s lawyers say turning boats back would be legal in limited and specific circumstances – although they have not confirmed what these will be.
Because the legal and safety risks are so high, it is also understood that Border Force chiefs have asked Ms Patel to personally support decisions to use the tactic, meaning she would have to be available to take a call from a Border Force vessel if and when they believe the tactic can be used safely.
The UK has a legal right to intercept people at sea where necessary to prevent crime or to protect borders.
But whatever the legal advice the UK government has received on migrant boats, it’s questionable whether such an operation could continue once there is a clear sign of danger.
Border Force commanders, like everyone else at sea, are under an international legal obligation to protect life where possible.
So their window of opportunity to run a push-back operation could be very tight.
But more importantly… is this really ever going to happen?
The intervention from the French interior minister may have just sunk the operation before a single Border Force ship has set sail.
There are no international waters in the Dover Strait – so any operation would need French cooperation.
And Gerald Darmanin’s statement makes clear France’s opposition to any confrontation of dinghies in one of the world’s most dangerous shipping lanes.