The UK will provide "vital military assets" to help a Nato mission tackling people smugglers in the Aegean, David Cameron confirmed as European leaders gathered to discuss the migrant crisis.
The Prime Minister, who will join counterparts from European Union nations and Turkey at a summit in Brussels, announced that the Royal Navy was deploying amphibious landing ship RFA Mounts Bay as part of the Nato deployment.
The ship, which carries a Wildcat helicopter, is expected to start operations in the coming days, spotting smugglers taking migrants to Greece and passing the information to Turkish coastguards so they can intercept the boats.
Two Border Force cutters will also join the operation, along with a third boat - the chartered civilian vessel VOS Grace - which is already in the Aegean.
RFA Mounts Bay will join naval vessels from Germany, Canada, Turkey and Greece as part of Nato's first intervention in the migrant crisis.
Mr Cameron said: "This migration crisis is the greatest challenge facing Europe today.
"Britain has not faced anywhere near the scale of migrants coming to Europe as other countries because we are out of Schengen and retain control of our borders.
"But where we can help, we should. And we've got to break the business model of the criminal smugglers and stop the desperate flow of people crammed into makeshift vessels from embarking on a fruitless and perilous journey.
"That's why this Nato mission is so important. It's an opportunity to stop the smugglers and send out a clear message to migrants contemplating journeys to Europe that they will be turned back.
"That's why the UK is providing vital military assets to work with our European partners and support this mission."
Around 1,800 migrants a day arrived in Greece in February, with more than 116,000 migrant arrivals across the Aegean already this year.
Downing Street said that at the EU summit Mr Cameron will call for work on breaking the link between people getting on a boat and being able to settle in Europe by "smashing" trafficking gangs and increasing the rate at which illegal migrants are sent back.
He will also urge support for Turkey, which already hosts 2.6 million migrants with many more sheltering on its border with war-torn Syria.
Greece should also be given help to speed up the processing of migrant claims and assistance in returning illegal immigrants to their countries of origin.
Liberal Democrat leader Tim Farron said: "It is a relief that the Government is finally firmly committed to search and rescue in the Med. It's just a shame it's not in time to save the 5,000 lives lost yearly to drowning on these terrifying journeys.
"Unfortunately, the smugglers will continue to win until refugees can access safe and legal routes, and in the meantime we cannot ignore the refugees who are already in Europe and in need of humanitarian relief.
"This is a step in the right direction, but there is still a long way to go."