After more than 40,000 Channel crossings so far this year, the two countries signed an agreement aimed at preventing people from making the perilous journey.
The UK negotiated a 40 per cent boost in the number of officers patrolling beaches in northern France.
British staff will also be embedded in French control rooms for the first time under the plans to clamp down on dangerous small boat crossings.
Further measures signed off in Paris include an investment in CCTV and dog detection teams to keep tabs on ports and plans to better equip officers with drones and night vision capabilities.
It will involve an increase in annual payments from Britain to France of £9 million, from £54m ($63m) to £63m ($74m).
The agreement was signed by Home Secretary Suella Braverman and French Interior Minister Gerald Darmanin on Monday.
After the deal, British Prime Minister Rishi Sunak could not say when the number of migrants crossing the Channel to the UK would be reduced.
Mr Sunak came under fire from critics and some of his own MPs, amid concerns that the latest agreement falls short of what is needed to curb the crisis.
“I’m confident that we can get the numbers down," he said before the G20 summit.
“But I also want to be honest with people that it isn’t a single thing that will magically solve this. We can’t do it overnight.
“But people should be absolutely reassured that this is a top priority for me. There’s lots more that we need to do.”
Mr Sunak said his earlier pledge for an annual cap on the number of refugees accepted in the UK was not his priority, as he was focused on tackling illegal migration.
“I think the first thing to do is to make sure that we can stop numbers of people coming here illegally," he said.
“Having a conversation like that can only happen after we’ve taken all the necessary steps to have proper control of our borders, so that there aren’t large numbers of people coming here illegally.
“And that is the priority, that’s the focus.
“And once we’ve done that, then obviously we can have a conversation about what is the right amount of people that we can safely and sustainably give refuge and sanctuary to.”
Rights groups are sceptical that the deal will address the underlying issues that lead to so many attempted crossings.
Steve Valdez-Symonds, Amnesty International's UK refugee and migrant rights director, criticised the deal for being "just the same as previous" ones, whereby money and resources are spent "on intercepting and obstructing people crossing the Channel, while doing nothing to address their need for safe access to an asylum system".
“The inevitable result will be more dangerous journeys and more profits led by ruthless smuggling gangs and other serious criminals exploiting the refusal of the UK and French governments to take and share responsibility," he said.
“Unless the UK government accepts its share of people into its asylum system, particularly people with family and connections in the UK, there seems little prospect that anything is going to change, let alone improve.”
An officer with the Union for Borders, Immigration and Customs (ISU), which is in charge of securing British borders, said the latest deal failed to address the “sticking points” behind the high number of migrants crossing.
Lucy Moreton told Times Radio that interrupting migrants to “just let them go to try again” would not have the required effect and nothing in the deal suggested that “the French are going to move away from that position”.
“The sticking points just simply have not been addressed” and France was unlikely to want to interrupt the flow of migrants out of their country, she said, calling on the UK to improve the resourcing of its court system to process claims faster.
Mr Sunak said he had raised the issue of migrant crossings in his first conversation with French President Emmanuel Macron and the pair “spent more time talking about it” during their meeting at the Cop27 climate talks in Egypt.
He said: “Only by working with other countries can you make progress on the things that impact people at home.”
Meanwhile, Britain and France have agreed to step up co-operation on the issue with European partners, with a meeting of the “Calais Group” of neighbouring countries to be scheduled as soon as possible.
A new task force will also be established to address the “recent rise in Albanians and organised crime groups exploiting illegal migration routes” into Western Europe, No 10 said.
Elsewhere, joint UK-France analysis teams will seek to boost information sharing.
Lastly, the deal pledges investment in French reception and removal centres for migrants who are prevented from making the crossing to the UK.
Ms Braverman said after signing the deal: “There are no quick fixes but this new arrangement will mean we can significantly increase the number of French gendarmes patrolling the beaches in northern France and ensure UK and French officers are working hand in hand to stop the people smugglers.”
Foreign Secretary James Cleverly said that efforts to tackle migrant crossings always need to “adapt and update”.
Migrants at immigration processing centre in Manston — in pictures
Speaking on Sky News on Monday, Mr Cleverly said: “We have got to get a grip of international trade in human misery.
“We do need to work with our international partners, including France and including the countries from which these people come.”
Asked how this new deal would differ from previous pacts, he said: “We need to always adapt and update.
“We see that these people traffickers constantly change their tactics when they use technology, how they try and evade law enforcement.
“So it’s important that we speak with our international friends and allies about updating our procedures. Also we need to make sure that these deals continue.”
Downing Street said the increase in the number of officers patrolling beaches in northern France would “increase early detection”, while the presence of UK staff in French control rooms would boost understanding of the “threat” at hand and help inform deployments.
The boost in port surveillance is designed to crack down on migrants attempting to enter the UK by lorry.
Enver Solomon, chief executive of the Refugee Council, claimed the deal failed to address the factors behind people choosing to put themselves at risk trying to reach Britain the first place — and will therefore “do little to end the crossings”.
He called for a focus on creating more “safe routes” and working with the EU and other countries to “share responsibility” for the “global challenge”.
Mr Sunak and French President Emmanuel Macron embraced at the UN climate change conference last week in their first direct encounter since he entered No 10.
Migrant crossings on the English Channel surge amid heatwave — in pictures
More than 40,000 migrants have crossed the Channel to the UK so far this year, according to government data, with 972 people detected on Saturday in 22 boats. In 2021, there were 28,561 recorded.
The arrivals on Saturday were the first since October 31 when 46 people were detected in one boat.
Immigration Secretary Robert Jenrick has detailed action the UK plans to take at home to tackle the small boat crisis.
The idea of “Hotel Britain” must be ended to deter “asylum shopping”, Mr Jenrick wrote in an article for the Sunday Telegraph.
Migrant children rescued in French waters — in pictures
Migrants are to be housed in “simple, functional” spaces as opposed to “luxury” rooms, he said, claiming the country’s “generosity” towards refugees was being “abused” by people “skipping the queue”.
The Home Office minister also said Britain’s modern slavery laws must not lead to exploitation by illegitimate claimants.
Mr Jenrick also said the government intended to “bust the backlog of asylum claims” by “cutting red tape” and introducing a pilot in Leeds that “doubled” the productivity of officials.