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"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 on Thursday.

The British and French leaders also aim to deepen cooperation in tackling terrorism at the meeting, as UK tries to strengthen ties before its exit from 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.

	The two leaders will meet at Sandhurt on Thursday [Getty]

"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.

A new treaty

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.

May is also expected to agree to welcome more young refugees stuck in Calais, a government spokesman said.

May and Macron will also announce enhanced police cooperation to control the border. 

Tackling extremists

The British prime minister is also set to commit to sending Royal Air Force (RAF) helicopters to a key French counter-terrorism operation in Mali. 

	The UK will also assist with France's counterterrorism operation in Mali and Africa's Sahel region [Getty]

The deployment of three RAF Chinook helicopters to provide logistic support to French troops tackling extremists across Africa's Sahel region is part of broader counter-terrorism and military efforts there by the UN, the EU and the African Union. 

"Recent terrorist attacks across Europe underline the scale of the cross-border challenge we face in keeping our citizens safe," the UK government spokesman said.

France in turn has agreed to commit troops to the British-led NATO battle group in Estonia in 2019.

Officials said it would build on the joint deployment of soldiers to the Baltic country whom the two leaders visited together last year.

At the summit the pair will also discuss their joint crackdown on online extremism "to ensure that the internet cannot be used as a safe space for terrorists and criminals," according to the spokesman.

Britain is also expected to allocate £50 million in additional aid for those affected by epidemics, natural disasters and conflict across Mali, Niger, Chad, North Cameroon, Burkina Faso and Mauritania.

The government hopes the cash will help provide 320,000 people with emergency food and provide protection for 255,000 refugees.

Thursday's gathering at Sandhurst military academy – the 35th UK-France summit – comes as Britain seeks to develop stronger bilateral ties with its continental partners ahead of leaving the EU in March 2019.

The issue of Brexit is not scheduled for formal discussion but will likely be touched upon in talks on other topics, the British official said. 

Summits in previous years have focused on defence and security, foreign policy and nuclear energy, but the 2018 agenda was broadened to cover "the full spectrum of the UK-France bilateral relationship including prosperity, innovation, science and education" he added.

Calais crackdown

Macron recently revealed his tougher-than-expected stance on immigration, vowing on Tuesday to enforce a zero-tolerance approach to migrant camps such as the notorious Calais jungle. He also warned those remaining in Calais with the intention to reach Britain that they were at a "dead end".

	Police brutality, including 'abusive tear gas use' against asylum-seekers has been reported in northern France [Getty]

Macron's announcements, which he made during a visit to Calais on Tuesday, drew criticism from some of his allies.

On Tuesday, his former senior aide Jean Pisani-Ferry penned a hard-hitting open letter along with several centre-left trade union and think-tank chiefs claiming Macron risked betraying his image as a humanist.

Writing in Le Monde newspaper, they urged him to "live up to our ideals" and put an end to efforts that seek to dissuade asylum seekers from coming to France in the first place.

In July, Macron declared that no refugee would face the winter on the streets or in the woods of Calais or he rest of France, however recent reports suggest that there is a real possibility of child refugees dying from cold.

There have also been findings of an "abusive use of tear gas" employed by French police against asylum-seekers in northern France.