Priti Patel is preparing to send back small boats carrying migrants in the Channel despite warnings from the French authorities that it could endanger lives.

Border Force staff are being trained to employ “turn-around” tactics at sea under plans developed for two years, a statement from the Home Office said.

It would allow UK officers to force small boats back into French waters. It is unclear if the proposals would include taking migrants back to French shores.

The proposals have already been rejected by the French government. A letter released on Wednesday showed they could not be accepted by the interior minister, Gerald Darmanin, on the grounds that “safeguarding human lives at sea takes priority over considerations of nationality, status and migratory policy”.

He has warned the UK that employing such tactics “would risk having a negative impact on our cooperation”.

The tactics will be used, the Home Office has said, with the intention of redirecting migrant vessels away from UK waters and back towards France.

Training, which is weather dependent, is due to conclude this month, with use of the tactics ready to deploy as soon as practical and safe.

Border Force has informed ministers they will only be able to deploy the tactics, which have been developed in consultation with maritime experts, when they deem it safe to do so.

In a statement, the Home Office said Patel had become “the first home secretary to establish a legal basis for the sea tactics, working with acting attorney general Michael Ellis and expert QCs.”

In a letter released after a bilateral meeting broke up on Wednesday, Darmanin made clear to Patel that the proposals would damage bilateral relations.

“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 refoulements [the practice of sending back asylum seekers] to French territorial waters would risk having a negative impact on our cooperation,” he wrote.

Conservative MPs have called for the home secretary to break international law and send all migrants arriving illegally by boat straight back to France.

Darmanin’s letter says that the French authorities have agreed with the UK to “double the number of personnel deployed on the Channel coast” and had been offered the use of a UK plane to monitor the coast.

Darmanin also rejected a suggestion from the UK to form a single force to stop people smugglers from using boats.

“Coordination between our forces on the coastline is, according to the teams themselves, good and effective. It does not require new structures to be created, as you propose, through a single, joint command centre for the forces.”

Earlier, the Home Office said 785 people crossed the Channel in small boats on Monday, short of last month’s record daily total of 828 migrants.

A record 13,500 migrants have crossed the Channel in small boats this year, including 1,000 in the past two days. Two hundred were prevented from crossing by the French on Monday, when 742 reached the UK.

Pierre-Henri Dumont, France’s MP for Calais, said earlier this week that turning around boats would be a blatant breach of human rights laws and an insult to the dignity of those seeking asylum.

“Not giving a chance to children to be protected is something that should not be tolerated in modern society. We are talking about human rights and dignity.

“This suggestion tears apart the UN Geneva conventions giving the right to everyone to apply to any country for asylum.

“The UK left the EU but the UK did not leave the international community and the UN,” he said.