Rishi Sunak and Emmanuel Macron further demonstrated their close working relationship by choosing to hold their UK-France summit talks in private.
It is understood the Prime Minister and the French president changed the original plans to ensure they could speak for longer without officials present at the Elysee Palace.
The once-regular summit is being held for the first time since Theresa May was in No 10, with the gathering in Paris seen as a chapter-change in cross-Channel relations after Brexit.
The Prime Minister’s official spokesman told reporters in Paris: “The Prime Minister and President Macron spoke for more than an hour one-to-one.
Close neighbours. Great friends. Historic allies. 🇬🇧🇫🇷
— Rishi Sunak (@RishiSunak) March 10, 2023
“They were joined at the end by their respective chiefs of staff for a brief period.
“It was a warm and productive meeting. They discussed the upcoming Six Nations match, having exchanged signed shirts.
“They both agreed on the need for further collaboration on a range of issues, including on defence, energy security and illegal migration.”
The No 10 official, asked why they opted to speak alone, said: “I think it is always beneficial for leaders to have good relationships and have detailed discussions on a range of issues
“There was a planned element for a one-to-one but they thought it helpful to extend that.”
The spokesman said the pair, who first met at the Cop27 climate summit in Egypt in November shortly after Mr Sunak entered Downing Street, spoke in English during the bilateral talks.
Commentators have dubbed Mr Sunak and Mr Macron’s cordial relationship as “Le Bromance”.
The two politicians are a similar age and have a background in banking.
Mr Sunak, speaking to reporters on the Eurostar journey to the French capital, said he saw France as a “close ally”.
The comments come after relations turned cold during Boris Johnson’s premiership, with rows over Brexit, fishing rights and the Aukus submarine deal between the UK, US and Australia.
During Liz Truss’s brief time as prime minister, she said the “jury is out” on whether Mr Macron was Britain’s friend.
Les destins du Royaume-Uni et de la France sont liés. Nos défis, partagés. Préservation de notre planète, soutien à l'Ukraine, coopération en matière de sécurité et d’énergie : c’est ensemble que nous avançons.
— Emmanuel Macron (@EmmanuelMacron) March 10, 2023
But her successor struck a very different tone when asked the same question en route to the summit.
He said: “My view is that having a strong collaborative relationship with the French, with President Macron, is a good thing for the country.”
After the behind-closed-doors discussions between the two leaders at the Elysee Palace on Friday, they met with French and British business leaders, including the heads of Octopus Energy and GlaxoSmithKline.
Their two teams ate lunch together, which included vegetarian sushi for starter, turbot and artichokes for main, followed by a praline tart for dessert.
The rugby shirts were exchanged ahead of England’s clash with France at Twickenham on Saturday as part of the annual Six Nations tournament.
They were signed by the respective playing squads of both teams, No 10 confirmed.
A communique setting out what has been agreed at the summit, which is being attended by a host of senior ministers from both countries, is due to be published after an afternoon press conference with the Prime Minister and the President.
Mr Sunak’s spokesman said the document would “talk about how both sides intend to do more on illegal migration”.
It follows reports that a deal could be agreed which is largely expected to build on a £63 million pact in November designed to clamp down on small boats crossing the English Channel.