The move has sparked claims that the agency has helped people enter the country illegally – and is understood to have angered Priti Patel, the hardline home secretary.
The Border Force is only meant to pick up migrants if they are in UK waters – and only if there is an immediate threat to life, which appears not to have been the case in the incident last Saturday.
It was revealed in a recording of a maritime radio conversation, obtained by the Daily Mail, which said UK and French officers discussed the “legality” of the operation.
The Valiant, a UK cutter, then entered the French side of the Channel and launched a fast inflatable boat to collect the migrants from a dinghy and bring them to Dover, the recording showed.
The move came ahead of a surge in Channel crossings as the summer weather arrived, with 585 migrants intercepted in just three days.
The French authorities dealt with eight incidents involving 130 people on Friday alone, while the UK dealt with four boats involving 83 people, the Home Office said.
Ms Patel is known to be frustrated that the French are not doing enough to stem crossings, but the country’s government has blamed Brexit for the increase in people making the perilous journey.
The UK ended legal routes for migrants to join family members in this country, forcing them to turn to traffickers and make attempts in small boats, it says.
A Home Office spokesperson said: “We are urgently investigating the circumstances behind this incident.”
The move by the Valiant appears to fly in the face of a statement to parliament, in January 2019, by then immigration minister Caroline Nokes .
“In the majority of cases, if a migrant is picked up in UK waters they are taken to the UK. If they are picked up in French waters they are taken to France,” Ms Nokes told MPs.
An action plan with France to curb the number of crossings meant that migrants found in the Channel would be returned to the “nearest safe port” in accordance with international maritime law.
The minister added: “Too often, migrants in the Channel dictate to those who come to their rescue which country they should be taken to. That is not right. I have asked immigration officials to do all they can to prevent asylum shopping at sea.”
Ms Patel has been criticised for increasingly draconian ideas to stem crossings, including putting floating barriers in the Channel.
She also looked into setting up a processing centre for asylum seekers on Ascension Island, 4,000 miles from the UK, or on disused ferries moored off the coast of England.
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies