Sir Keir Starmer and Emmanuel Macron met at No 10 as the UK pushes for France to do more to stop migrant crossings.
Mr Macron said a renewed partnership between the two countries on border security and other areas was “a game changer”.
The French president arrived on Tuesday for the first state visit by an EU head of state since Brexit.
It comes as the UK has been pressing for tougher action from the French authorities on the beaches along the Channel coast.
The Prime Minister hopes to strike a “one in, one out” deal to send small boat migrants back to the continent, in exchange for the UK accepting asylum seekers in Europe who have a British link.
At Downing Street, the French President said he believed the state visit and upcoming Franco-British summit on Thursday played a "unique role in these challenging times", particularly for action regarding Ukraine.
He said the coalition will help "build stronger links and make big moves forward on defence, security, technology, economy" as well as "more exchange on culture, education, research and when we look at climate change, AI, but as well as challenging times from Ukraine to Middle East.”
"The partnership and the friendship between UK and France is absolutely, I would say, a game changer for not just Europe, but a lot of these issues," the president added.
Mr Macron thanked Sir Keir Starmer and the King for their hospitality so far during his three-day state visit.
Sir Keir said the meetings were "hugely symbolic of the closeness between our countries and our personal relationship".
He added: "Whether it's on defence and security, whether it's on Ukraine, whether it's on trade and economy, business opportunities, capabilities, we think alike, we work together, and I feel this relationship is as strong as it's ever been."
At Prime Minister's Questions, Sir Keir highlighted the joint work being done on cross-Channel migrants.
Liberal Democrat leader Sir Ed Davey challenged him to stop handing over money to the French unless they agreed to "a returns deal and doing their bit to stop the boats".
Sir Keir told him: "We're working closely with the French on this issue, we'll only provide funding that delivers for our priorities.
"We are working closely. We share information to a much greater extent than was the case before. We've got a new specialist intelligence unit at Dunkirk, and we're the first government to persuade the French to review their laws and tactics on the north coast to take more effective action."
Reform UK leader Nigel Farage told Sir Keir: "The country demands that you say to the French president 'we will not accept undocumented males across the English Channel' and that you are not dictated to by an increasingly arrogant, anti-Brexit French president."
In a speech to MPs and peers on Tuesday, Mr Macron promised to deliver on measures to cut the number of migrants crossing the English Channel, describing the issue as a “burden” to both countries.
He said France and the UK have a “shared responsibility to address irregular migration with humanity, solidarity and fairness”.
Decisions at a Franco-British summit on Thursday will respond to “our aims for co-operation and tangible results on these major issues”, Mr Macron added.
But he also stressed that Britain and France would “only arrive at the lasting and effective solution” if they addressed the “pull factors” encouraging people to make the journey across the Channel.
On Wednesday, Cabinet Office minister Nick Thomas-Symonds insisted that the UK was already working to reduce “pull factors”.
He told ITV’s Good Morning Britain: “That is why this Government is cracking down on illegal working and why there have been over 7,000 arrests in that space.”
Mr Thomas-Symonds declined to give a “running commentary” on negotiations with the French on migration, but told Times Radio that work with Paris was already “bearing fruit”.
The French denied a Telegraph report that Mr Macron blames the UK for the crisis.
A senior Elysee source said: “The French president looks forward to working with the Prime Minister constructively on this shared priority.”
Last week, the total number of people crossing the Channel in small boats this year passed 20,000.
The total now stands at more than 21,000, a record for this point in the year.
Sir Keir and the French president are also expected to co-host a meeting of the “coalition of the willing”, the peacekeeping mission proposed to be deployed to Ukraine in the event of a ceasefire with Russia.
Meanwhile, French multinational Veolia announced plans to build Britain’s first closed-loop recycling facility to turn Pet trays into food grade materials in Shropshire, which the firm said would create 130 permanent local jobs.
The £70 million investment will help 80,000 tonnes of plastics get processed into new packaging each year, the company said.
