Britain said it would pay an extra 44.5 million (50 million euros, $62 million) to boost security around Calais following a demand for more money from President Emmanuel Macron ahead of a key summit today.
"This is about investing in and enhancing the security of the UK border," a government spokeswoman said.
The funding will go towards fencing, CCTV and detection technology in the northern French port city as well as at other points along the Channel from which migrants regularly attempt to reach British shores by ferry or train.
The money would be on top of more than 100 million already paid by Britain following a border deal between the two countries that has now been renegotiated and is due to be signed today.
British and French leaders also aim to deepen cooperation in tackling terrorism at the meeting, as Britain tries to strengthen ties before leaving the European Union next year.
Macron, who is on his first official trip across the Channel, will meet May at an army base near the British capital.
In a piece of diplomatic theatre, Macron is expected to confirm that France will agree in principle to loan London the Bayeux Tapestry, the famed 941-year-old embroidery that recounts the Norman conquest of Britain in 1066.
"Today's summit will underline that we remain committed to defending our people and upholding our values as liberal democracies in the face of any threat, whether at home or abroad," May said in a statement ahead of the talks.
"Our friendship has always gone far beyond defence and security and the scope of today's discussions represents its broad and unique nature," she added.
The leaders are expected to focus on the sensitive issue of immigration.
Hundreds of people continue to camp out in the northern French town, hoping to stow away on trucks heading to Britain, a destination seen as an El Dorado by some migrants from Afghanistan and parts of Africa.
The two countries currently abide by the 15-year-old Treaty of Le Touquet, which permits immigration checks within each other's borders.
A new treaty will be signed at Thursday's summit to complement the 2003 deal, according to French officials.
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