David Cameron is to urge fellow EU leaders to make good on promises of cash to help Turkey cope with migrants passing through the country in the hope of reaching Europe, Defence Secretary Michael Fallon has said.
EU leaders are meeting Turkish Prime Minister Ahmet Davutoglu at a special summit in Brussels, where they are expected to announce the closure of the Balkan route used by migrants to travel to northern European states after their arrival in Greece.
The UK has announced plans to deploy a Royal Navy ship to join a Nato operation to tackle people smugglers bringing migrants across the Aegean Sea.
Mr Fallon said the amphibious landing ship RFA Mounts Bay will use an onboard helicopter to provide information for the Turkish coastguard about the routes being used by smuggling gangs, giving them a better chance of intercepting boats attempting the perilous crossing to Greece.
The EU last year agreed a 3 billion euro (£2.3 billion) facility to help Turkey deal with the refugee crisis which has seen 2.6 million migrants enter the country, many of them fleeing the civil war in neighbouring Syria.
Mr Fallon said Mr Cameron will tell other EU leaders that the money now needs to be delivered more quickly.
The Defence Secretary told the BBC Radio 4 Today programme: "The Prime Minister will be making exactly that point at the meeting in Brussels today. Europe has promised Turkey aid and that now needs to be delviered and the coastguard there needs to be strengthened and we need to do as much as we can to help Turkey."
The amphibious landing ship Mounts Bay, which carries a Wildcat helicopter as well as Royal Marines and medics, is expected to start operations in the coming days, joining naval vessels from Germany, Canada, Turkey and Greece as part of Nato's first intervention in the migrant crisis.
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.
Mr Fallon said: "The primary mission is to build up a proper picture of the smuggling routes. Obviously if there are migrants at sea, the law of the sea dictates that the nearest vessel must pick them up. But the first thing is to build up a picture of these routes and to start breaking the smugglers' business model.
"Smugglers are making money out of people drowning now. We've had several hundred drowned this winter, several thousand drowned this year, and what's essential is to work out where this people smuggling is done from and then to get a policy in place of returning people, which in the end will stop people making this very dangerous crossing.
"We have to put a stop to this because otherwise we are going to see more lives lost and more misery."
Mr Fallon said Turkish coastguards and Greek authorities were becoming "overwhelmed" by migrants coming not only from Syria but also from Afghanistan, Pakistan and further afield.
And he said that a key to stemming the flow will be to return more of the migrants to Turkey after their arrival in Europe.
"What's not happened so far anywhere in Europe is that people haven't been returned," he said. "Once they start being returned then there is less prospect of people paying money and smugglers making money out of what is a very dangerous crossing."
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.
Mr Cameron described the migration crisis as "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," he said.
"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."
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."