After hosting Macron for a three-day visit that included a carriage procession to Windsor Castle with King Charles and a state banquet, Starmer wants Macron to deliver on his promise of greater "co-operation and tangible results" on migration.
Starmer, who has faced challenges to his popularity since his election landslide in 2024, is working to address high levels of immigration, including asylum seekers arriving by small boats, to try to stem the growing influence of the Reform UK party, led by Brexit campaigner Nigel Farage.
Before the start of Thursday's summit, Starmer, sitting next to Macron at his Downing Street office, said: "We all agree the situation in the Channel cannot go on as it is."
"We're bringing new tactics into play and a new level of intent to tackle illegal migration and break the business model of a criminal gang."
Macron said the two countries "shared the same resolve to fight against illegal criminal gangs, with strong coordination with other EU states".
A British government source said Britain and France had agreed to a deal on a "one in, one out" migrant returns scheme - which would see Britain deporting to France undocumented people arriving in small boats, in return for an equal number of legitimate asylum seekers with UK family connections.
The government source said it would initially be limited, but could be scaled up.
More than 21,000 people have arrived on small boats in 2025, a record number.
The importance of the deal will be highlighted as a change in efforts to tackle migration by Starmer, who, like Macron, is facing domestic woes, but it was not clear whether the agreement would come with conditions or have a big impact.
The policy, which is similar to a scheme used by the EU and Turkey, carries risks for Macron from his right-wing political critics who might question why he has agreed to take back migrants wanting to live in Britain.
Macron had also called on Britain to address "migration pull factors", suggesting that it should be harder for migrants in Britain to find work without legal residential status.
Underlining closer ties between the two countries since Britain left the European Union in 2020, the two leaders will strengthen their defence ties.
Both pledged to order more Storm Shadow cruise missiles, now used in Ukraine, and signed an agreement to deepen their nuclear co-operation, which will say for the first time that the respective deterrents of both NATO allies can be co-ordinated.
The agreements come after both nations spearheaded a "coalition of the willing", a group of countries that plan to support Ukraine in the event of a ceasefire with Russia.
