The British Prime Minister Keir Starmer and the French President Emmanuel Macron will hold talks as the UK pushes for France to do more to stop migrant crossings.
The French president arrived for the first state visit by an EU head of state since Brexit yesterday.
It comes as the UK has been pressing for tougher action from the French authorities on the beaches along the Channel coast.
The UK 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.
Alongside Downing Street talks, the leaders are also expected to attend a reception with UK and French businesses and an event at the British Museum.
Their spouses, Brigitte Macron and Lady Victoria Starmer, will have tea and a tour of Downing Street together, followed by all four having lunch.
While they are being hosted by King Charles at Windsor, the Macrons will lay flowers on the tomb of Queen Elizabeth II and see a horse the French president gave her in 2022 to mark her Platinum Jubilee.
In a speech to MPs and peers yesterday, 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 tomorrow will respond to "our aims for co-operation and tangible results on these major issues", Mr Macron added.
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.
The leaders 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.
