Boris Johnson has met French President Emmanuel Macron in Downing Street to mark the 80th anniversary of a famous wartime broadcast.
In 1940, French Resistance leader Charles de Gaulle used the BBC to send a radio message to Nazi-occupied France, urging people not to give up the struggle against Hitler.
Mr Johnson praised the "courage and sacrifice" of those who fought on.
The Prince of Wales and Duchess of Cornwall welcomed Mr Macron to the UK.
And the Red Arrows and their French counterparts, La Patrouille, performed a flypast above London to mark the occasion.
During their meeting, Mr Johnson and Mr Macron discussed post-Brexit trade arrangements between the UK and EU, with No 10 saying the prime minister "welcomed the agreement to intensify talks in July".
A spokesperson added that he had restated that the UK "does not believe it makes sense for there to be prolonged negotiations into the autumn". The "transition period" - during which the UK remains in the EU single market and customs union - is due to finish at the end of the year.
The two leaders also talked about easing the 14-day coronavirus quarantine measures in place for visitors to - and UK citizens returning to - the UK.
Mr Macron was exempt from the requirements, as a "representative of a foreign country on business".
It was earlier announced that four surviving French Resistance fighters are to be appointed honorary MBEs.
Edgard Tupet-Thome, 100, Daniel Bouyjou-Cordier, 99, Hubert Germain, 99, and Pierre Simonet, 98, are already members of the Order of Liberation, an honour given by France to those who played an outstanding role in freeing the country from its four-year wartime occupation.
Mr Macron's visit also comes after it was announced that Dame Vera Lynn, the singer dubbed the "forces' sweetheart" for her morale-boosting performances during World War Two, has died aged 103.
The two leaders viewed artefacts and letters from General de Gaulle's time in London and from his partnership with the UK's wartime prime minister, Sir Winston Churchill.
Mr Johnson presented Mr Macron with a framed montage of a telegram sent from General de Gaulle to Sir Winston on VE Day, in 1945, and Sir Winston's reply.
He also gave him a model of Sir Winston's open-top Land Rover and a photograph of General de Gaulle in Paris, shortly after the city's liberation from German forces in 1944.
A key date in UK-French history
By Gavin Stamp, BBC News
It was 80 years ago that General Charles de Gaulle broadcast a historic message from London to his fellow countrymen imploring them not to give up the fight against Hitler.
France was on its knees at the time, German troops having entered Paris four days earlier, and on the verge of agreeing an armistice confirming its formal military surrender.
In the message, broadcast in French, De Gaulle said: "Whatever happens, the flame of the French resistance must not be extinguished and will not be extinguished."
Transmitted on the BBC's French Service, the broadcast was not recorded and relatively few people in France heard it.
But a similar broadcast four days later on the same network reached a wider audience and went a long way to establishing de Gaulle as his country's leader in-exile.
The 18th June 1940 remains one of the most important dates in UK-French history and still has enormous resonance on both sides of the channel.
After the fall of France, General de Gaulle made a speech from London 18 June 1940. Known as the "Appel", or appeal, it rallied the country in support of the Resistance.
Mr Macron, speaking next to General de Gaulle's statue in Carlton Gardens, in central London, said: "This is where de Gaulle was able to call on the French people to join the Resistance, the soldiers of the shadows.
"Because 80 years ago today, on June 18 1940, the United Kingdom gave Free France its first weapon, a BBC microphone."
Mr Johnson said General de Gaulle had arrived in 1940 knowing that Britain and France's shared values of "freedom, tolerance and democracy" were under threat and pledging to defend them.
"The struggles we face today are different to those we confronted together 80 years ago. But I have no doubt that - working side by side - the UK and France will continue to rise to every new challenge and seize every opportunity that lies ahead," he said.