Castex wrote to Johnson on Wednesday setting out France’s suggestions to stop migrants crossing the Channel in small boats following a tragedy last week in which at least 27 people drowned.
He formally rejected an idea proposed by Johnson of British security forces patrolling on the French coast to prevent dinghies taking to the water.
“We cannot accept… that British police or soldiers patrol on our coasts.
“It’s a question of sovereignty and I know your government’s sensitivity towards respecting the sovereignty of others,” Castex wrote.
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The letter did not directly address another controversial idea of Johnson’s of returning all migrants to France that cross the Channel by sea, which the British prime minister believes “would significantly reduce – if not stop – the crossings.”
It said France would work towards a migration agreement between Britain and the European Union “which could include a virtuous transfer mechanism.”
“Sending migrants back to us is not an option and is not a serious or responsible way of tackling the issue,” one of Castex’s aides said on Thursday, asking not to be named.
Franco-British relations, already seen as at their lowest in decades, dived again after the mass drownings on November 24th.
Johnson made proposals in a letter to French President Emmanuel Macron last week that were seen in Paris as an attempt to deflect blame onto France.
His decision to make the letter public before Macron had read it was seen as a breach of diplomatic protocol, with the French president later condemning his approach as “not serious.”
In retaliation, an invitation to British Home Secretary Priti Patel to take part in a meeting of European ministers in France at the weekend to discuss migration was withdrawn.
According to the Canard Enchaine newspaper on Wednesday, Macron called Johnson a “clown” as well as a gougnafier (knucklehead) in private conversations with aides last week.
“It is a pretty unhelpful word,” UK Business Minister George Freeman told Sky News on Thursday.
“Of course, the Prime Minister isn’t a clown, he is the elected prime minister of this country with a very big mandate, leading this country through the pandemic.”
Castex’s letter to Johnson, shared with journalists a day after being sent, was strongly worded, but began by saying that “every country must face up to its responsibilities” in tackling the Channel crossings.
It said that France was deploying 700 police officers to patrol its northern coast, while 41 people-smuggling rings had been broken up since the start of the year and 1,552 suspected smugglers had been arrested.
It added, however, that “managing the reception of migrants that want to go to your country falls firstly on France, which is not normal.”
It also acknowledged that fellow European Union members such as Belgium, the Netherlands and Germany needed to do more to tackle people-smuggling and illegal migration.
But it stated that “a large part of the solution is not to be found in France, but in the United Kingdom.”
Castex urged Britain to adopt “a more efficient returns policy” to deport failed asylum seekers, as well as opening up legal migration routes for “those who have legitimate reasons to want to come to your country.”
“Only you can ensure that your labour market is sufficiently controlled to discourage people wanting to work illegally,” he wrote.
It also warned Britain against pushing back migrant boats, an option that has been under discussion by the government in London, saying this would “endanger the lives of migrants and would break maritime law.”
In addition, France wanted improved intelligence-sharing from the UK, particularly to bolster a shared intelligence centre in northern France.
“We have noticed that Britain supplies it with little,” the prime minister’s aide said.
A second aide denied that France was seeking to shift responsibility for the crossings on to London.
“We are not approaching this as a blame-game. We’re approaching it as a shared responsibility,” the aide said.
France is regularly criticised by rights groups for denying entry to asylum seekers on its southern border with Italy.