British Prime Minister Boris Johnson has no regrets about publishing a letter to President Emmanuel Macron on Twitter proposing that France take back migrants who have crossed the Channel, his spokesman said Friday.

France has called the move "unacceptable" and cancelled Britain's attendance at planned talks between European Union interior ministers on the issue set for Sunday.

But asked by reporters at a daily briefing if Johnson regretted sending and then sharing the letter on social media, his spokesman said "no".

"As the PM (prime minister) has said himself, both he and President Macron have a shared recognition of the urgency of the situation we're both facing," he said.

"You'll see from the tone of the letter this is about deepening our existing cooperation and building on the work that's already been done between our two countries, which the PM in the letter paid tribute to."

Macron earlier told a press conference in Rome that he was "surprised" by Johnson's methods, which he branded "not serious", adding: "One leader does not communicate with another on these questions on Twitter, by public letter".

However, defending the decision to tweet, the UK leader's spokesman insisted the British public "would rightly want to know what we are looking at in terms of trying to solve this problem".

Relations between the two neighbours were already seen as their most tense in decades following a series of disputes over Brexit.

But the spat represents a further turn for the worse.

It comes as the crisis of migrants illegally crossing the Channel intensifies after 27 people died in the busy waterway earlier this week.