Theresa May has welcomed President Emmanuel Macron for summit talks which saw Britain commit tens of millions of pounds to strengthen UK border controls in France, as well as the signing of a new treaty designed to improve security in Calais.
Mr Macron had reportedly pressed the Prime Minister in recent weeks to increase funding for security and to take in more refugees.
While more money for security was pledged during the meetings, no promises were made on the numbers of refugees taken in by Britain.
During a news conference at the Royal Military Academy in Sandhurst, following the conclusion of talks, Mr Macron indicated he would stand firm on the issue of financial services during Brexit talks, saying he was "here neither to punish nor reward" the UK following its decision to leave the European Union.
During Thursday's meetings, Home Secretary Amber Rudd and French Interior Minister Gerard Collomb signed the Sandhurst Treaty, which as well as improving security in the French port city, seeks to accelerate the processing of migrants seeking to come to the UK via Calais, with a reduction from six months to one month for adults and 25 days for children.
The treaty also seeks to do more to address the issues which are causing people to try and come to the UK, in what Mrs May described as "tackling the issue upstream".
The talks also saw the UK commit £44.5 million to strengthen UK border controls in France, with the cash going towards fencing, CCTV and detection technology in Calais and other ports along the Channel.
"This is about investing in and enhancing the security of the UK border,” a government spokeswoman said.
"Just as we invest in our borders around the rest of the UK, it is only right that we constantly monitor whether there is more we can be doing at the UK border controls in France and Belgium to ensure they are as secure as possible."
The pair also agreed that both countries remain committed to the Le Touquet agreement, which allows the UK and France to post border officials on one another's soil.
The French President's visit to the UK was his first since entering the Elysee Palace.
The talks also witnessed the first meeting of the heads of the two countries' main intelligence agencies, with MI5, MI6, GCHQ and France's DGSE and DGSI in attendance to discuss collaborating in the fight against terrorism and increasing cooperation on defence.
Watch the press conference in full
Britain also committed to Mr Macron's "European intervention initiative," a plan to better coordinate armed forces from different nations.
Mrs May confirmed the UK had agreed to step up support for French operations against so-called Islamic State in the Sahel region of Africa, with three UK Chinook helicopters and their crews deployed to the area.
Meanwhile France said it would commit troops to a UK-led battle group deterring Russian aggression in Estonia.
Political Correspondent Carl Dinnen discusses the talks about Brexit which were held as part of the meeting
On Brexit, Mrs May restated her call for a deep and special partnership and comprehensive trade agreement between the UK and EU after Brexit, highlighting the large amounts of trade Britain and France do with each other.
"We recognise that as we leave the EU we will no longer be full members of the single market," the Prime Minister said.
"We recognise that. There will be a different relationship in future, a different balance of rights and responsibilities, and we've been very clear about that.
"But I believe that it is actually in the interest not only of the United Kingdom, but also the European Union as it goes forward, to continue to have a good economic relationship and partnership with the UK and I believe that should cover both goods and services."
Meanwhile Mr Macron said he wanted the single market to be "preserved because that is very much the heart of the EU.
"I want to make sure that the single market is preserved because that is very much the heart of the EU.
"The choice is on the British side, not on my side. But there can be no differentiated access for the financial services.
"If you want access to the single market - including the financial services - be my guest. But it means that you need to contribute to the budget and acknowledge European jurisdiction.
The pair also discussed the "very significant" proposals by the French to loan the Bayeux Tapestry to Britain, where it has never been displayed, agreeing that it would come to the UK in 2022 and help forge cultural and scientific links.
Mr Macron's focus on border issues, however, was underlined by a visit to Calais on Tuesday when he vowed there would be no return of the infamous migrant "Jungle" tent city which was cleared by the authorities in 2016.
UK funding for security in the region is thought to have topped £100 million over the last three years, while more than 750 children have been transferred to Britain since the Jungle was shut down.
British officials said the new UK funding would build on previous security work in the area, pointing to figures showing illegal attempts to enter the UK fell from more than 80,000 in 2015 to just over 30,000 last year.
Up to 7,000 men, women and children lived in the Jungle in Calais before the site was cleared in 2016. But hundreds of asylum seekers hoping to cross the Channel remain in the area, more than a year after authorities dismantled the sprawling site.
Speaking ahead of the meeting, Mrs May said the summit underlined the "broad and unique nature" of the relationship between the two countries.
"Today's summit will underline that we remain committed to defending our people and upholding our values as liberal democracies in the face of any threat, whether at home or abroad," she said.
"What is clear from the discussions we will have today is that a strong relationship between our two countries is in the UK, France and Europe's interests, both now and into the future."