France has formally rejected the Prime Minister's call for British authorities to conduct joint patrols on the beaches around Calais to deter migrants from crossing the Channel.
In a letter to Boris Johnson, French Prime Minister Jean Castex said "we cannot accept" the presence of British police officers or soldiers as it would compromise the nation's sovereignty.
He also suggested the UK reform its systems to offer "legal immigration paths" for people to come to the country instead of risking the perilous crossing.
However, he did promise that France would examine "in good faith" some of the proposals put forward to resolve the crisis.
The UK Government has promised to work in "close co-operation and partnership" with France in the wake of Mr Castex’s letter.
According to Le Monde, Mr Castex wrote: "We have always accepted to examine and discuss in good faith British proposals of reinforcement and cooperation.
"We have accepted some, we have declined others."
Mr Johnson had suggested Border Force officers, or failing that private security contractors, could be deployed in joint patrols.
Mr Castex sad: "We cannot accept, for example, that British police officers or soldiers patrol our coasts.
"It comes from our sovereignty."
Watch: The Royal Navy was deployed to patrol the English Channel amid migrant concerns in January 2019.
France has repeatedly turned down British requests for joint land and maritime operations in its territory.
The French prime minister said more than 700 police officers and gendarmes were already covering the area around Dunkirk and Calais to prevent small boats carrying migrants taking to the water.
But these efforts "only permit us to contain the phenomenon, not to bring a lasting response".
To do that, he suggested the UK must open legal immigration paths for those who have legitimate reasons to enter the country, and pursue a "more efficient" returns policy for those who do not.
A UK Government spokesman said: "Last week's devastating events were a tragic reminder of the dangers of these crossings and like our French neighbours the UK Government is determined to prevent further loss of life in the Channel.
"We stand ready to discuss all options in the spirit of our close cooperation and partnership, and as a shared, global challenge it is vital we address illegal migration collectively and urgently."
It comes after Mr Johnson sparked fury in France by publishing his letter to President Emmanuel Macron calling for further action in the wake of the tragedy which saw 27 people lose their lives while attempting to cross the Channel in November.
The bitter feud has seen reports that Mr Macron labelled Mr Johnson a "clown" and a "knucklehead".