Boris Johnson has written to Emmanuel Macron formally proposing joint patrols on French beaches as early as next week to stop small boats carrying migrants heading for the UK.
In a letter spelling out UK plans after 27 people drowned in the Channel, Mr Johnson also calls for a new “returns agreement” between the two countries.
The deal, if agreed by Paris, would see migrants who arrive on UK shores illegally sent back to France, with London taking in more child migrants with British family links in return.
Downing Street sees the pact as the best long-term solution to the escalating problem of boat crossings, as it would undercut smugglers promising a route into the UK.
A senior government source said: “A returns agreement would be the single biggest deterrent of these crimes. If migrants that cross the Channel illegally are sent back to France this totally busts the smuggling gangs.”
Other UK proposals include more sensors on French beaches, better airborne surveillance, extra maritime patrols and deeper joint intelligence work.
The letter will be seen as an attempt to push ahead with constructive discussions after days of finger-pointing between British and French politicians following the tragedy in the Channel.
Emmanuel Macron, the French President, urged Mr Johnson on a call on Thursday night “to refrain from exploiting a tragic situation for political ends”, according to an Élysée Palace readout.
Hours earlier, Mr Johnson had called on France to “step up” and do more with the UK to stop small boats, saying “we’ve had difficulties persuading some of our partners, particularly the French, to do things in a way that we think the situation deserves”.
The discussions are complicated by tensions in the relationship on other issues, such as the number of UK fishing licences granted to the French, a stand-off over trade tensions in Northern Ireland caused by Brexit and the recent Australia-UK-US nuclear submarine pact.
French fishermen are set to blockade the port of Calais and the Channel Tunnel on Friday amid the row over licences to fish in British waters.
Cabinet ministers are among those in the UK who believe Mr Macron has lent into recent political rows to shore up support at home ahead of his re-election bid next spring - an accusation of politicisation that French ministers have also thrown at Downing Street.
There were signs on Thursday of talks becoming more constructive. Home Office officials will travel to France on Friday for discussions on wider cooperation between the two countries on Channel migrants.
Priti Patel, the Home Secretary, is expected to attend a meeting of EU immigration ministers on Sunday to discuss measures to tackle the migration crisis facing Europe.
France is concerned at the number of Channel migrants entering through Belgium, often on the day of their intended crossing, exploiting the EU’s open borders.
Mr Johnson’s letter doubles down on Ms Patel’s offer - made last week and repeated on Thursday - for UK police, border officials or even troops to patrol French beaches.
There is concern within Whitehall that French officials have not been doing enough to stop boats of people seeking asylum in the UK being launched from their shores.
The Prime Minister is proposing that Border Force officials join the French for such patrols as soon as possible, potentially as early as Monday.
It is expected they would not be able to carry out arrests, but could help monitor potential launching points and deter smugglers.
It is unclear if the French will agree to the suggestion. Some French politicians on Thursday said the idea could undercut French sovereignty.
Another option Downing Street is open to is using private security to help with patrols. Home Office sources downplayed the likelihood that British troops would be involved.
Plan to dismantle people-smuggling networks
Speaking during a visit to Croatia on Thursday, Mr Macron said he would hold the UK to account and reiterated his call for more help in fighting the smugglers.
“We are going to ask for extra help from the British because these men and these women don’t want to get asylum in France” despite there being centres where they could apply in Calais and Dunkirk, he said.
“Basically we have got to develop [relations] with our partners and hold them to account. We’ve got to develop things in a far stronger way, we’ve got to reinforce cooperation - co-operation [with] Belgium, Holland, Germany, but also Britain and the [European] Commission.”
France wanted to work more closely with Britain to dismantle people-smuggling networks, the president added.
Jean Castex, his prime minister, also held a crisis meeting on Thursday with ministers to discuss new measures and invited the British, Belgian, Dutch and German immigration ministers to a meeting in Calais on Monday.
The talks would aim to “better combat the networks of smugglers who are behind these migration flows”, insisting that a response on a “European scale” was needed, his office said.
Both sides agree that more must be done to smash smuggler networks, many of which are based in the UK, according to French interior minister Gerald Darmanin.