France rejects joint Channel border patrol with Britain in letter to Boris Johnson

French Prime Minister Jean Castex told his British counterpart Boris Johnson that he rejected the idea of a joint British-French patrol in the English Channel to fight migration to Britain, a letter obtained by Reuters showed on Thursday.

"France is ready to pursue our operational cooperation with the United Kingdom", the letter said, but added: "We cannot accept ... British police or military patrol on our coast. It's a matter of our sovereignty."

Johnson last week suggested a joint coast patrol in a letter he sent to French President Emmanuel Macron - and later published on Twitter - after 27 migrants lost their lives in the Channel as they tried to reach British shores.

Johnson's tweet caused outrage in France, leading to Macron telling London to "get serious".

"I'm surprised when things are not done seriously. We don't communicate between leaders via tweets or published letters, we are not whistle-blowers. Come on. Come on," Macron later told a news conference in Rome.

Relations between the traditional allies are already strained, including by a recent submarine deal with Australia which replaced one it had with France, and a conflict about post-Brexit fishing licences.

In Thursday's letter, Castex reiterated that it was up to Britain to resolve the ongoing issue.

"A large part of the solution does not lie in France, but in the United Kingdom. In fact, you can deter the migrants who are not destined to settle [in Britain] from coming to your territory by conducting a more effective return policy."