The UK will pay France £8m more a year under a revised deal to try to stop people crossing the English Channel in small boats.
The money will pay for increased surveillance of French beaches, while UK police officers will also be able to observe patrols within France.
It is thought French officers patrolling the coast will rise from about 250 to 350 over five months.
PM Rishi Sunak said he was "confident" the crossings could be brought down.
However, he warned there was no "single thing" that could "fix" the situation, promising "even greater cooperation" with France in the months ahead.
Labour leader Sir Keir Starmer called the deal a "small step in the right direction," but said more needed to be done to tackle people smuggling.