The French president had threatened to abandon the Le Touquet agreement, which allows British border guards to patrol on French soil, after Brexit. Such a move would have risked tens of thousands more migrants arriving illegally every year, rather than be stopped in France before they can board ferries or enter the Channel Tunnel.
On Thursday, France will renew the Le Touquet deal and the Prime Minister will agree to take more of the unaccompanied child migrants who flock to France in hope of reaching the UK.
Leading Brexiteers on the Conservative back-benches have said that giving more money to France is “absurd” since policing the Channel ports is a matter for the French and the bill could be taken out of the £39 billion Brexit divorce payment.
The Government insisted the deal will save hundreds of millions in the cost of processing illegal migrants and asylum seekers once in Britain.
Mr Macron, making his first presidential visit to the UK, will meet Mrs May at the Royal Military Academy in Sandhurst, Berkshire, chosen because the summit is focused on military and security cooperation.
History will also be made when the heads of all five British and French intelligence services meet for the first time. They will agree to take intelligence sharing to “the next level” to tackle jihadists dispersing from Iraq, Syria and Libya to regroup and plan attacks on the UK and France.
The heads of MI5, MI6, GCHQ and France’s DGSI and DGSE – the equivalents of MI5 and MI6 – will also discuss better ways to “ensure that the internet cannot be used as a safe space for terrorists and criminals”.
Mr Macron, who said during his election campaign that France could no longer be Britain’s coastguard, forced the issue of border security on to the agenda. Mrs May, who is desperate for allies in the second phase of Brexit talks, agreed to pay £44 million for security fencing, CCTV and detection technology at Calais and other ports.
The Elysée has hailed the extra money as a victory for Mr Macron, but Whitehall sources insisted it would result in savings for Britain. In response to French claims that Mrs May had agreed to take in more unaccompanied child migrants, Government sources said the overall number allowed in under the so-called “Dubs amendment” would remain at 480, but more might now come from France rather than countries such as Greece and Italy.
Since 2014, Britain has given around £140 million to France for infrastructure at its Channel ports. However, the Home Office said extra border security since 2015 had reduced the number of recorded attempts by illegal migrants to enter the UK from 80,000 to just over 30,000 per year.
Charlie Elphicke, the MP for Dover, said: “It’s not right to hand over yet more money to France. It’s now nearly €200 million … and it’s too much.”
Mr Macron will go for a private lunch with Mrs May at a gastro-pub in her Maidenhead constituency before moving on to Sandhurst where the president will be given a guard of honour.
They will later attend a reception at the Victoria & Albert Museum.
Mrs May will also agree to military assistance for French counter-terrorism efforts in Mali, including three Chinook helicopters and crew. The heads of the intelligence services will discuss stamping out “safe spaces” in Africa where jihadists can plan attacks.
Mrs May’s charm offensive ahead of fresh Brexit talks will also include cultural exchanges, including 10 new towns to be twinned with French counterparts, an increase in student exchange schemes and a promise of more French being taught in British schools.
Mr Macron will also formally agree to the loan of the Bayeux Tapestry to Britain in around five years’ time, subject to curators agreeing that it can be safely transported. Britain is prepared to loan France one of its own cultural treasures, possibly the British Museum’s Rosetta Stone.