David Cameron has welcomed an EU deal with Turkey, which he said could "significantly" reduce numbers of migrants crossing the eastern Mediterranean to enter Greece by boat.
Under the terms of the agreement, Greece is to start returning migrants arriving by boat from Turkey from the beginning of next week.
The deal marks a radical shift of tactics after a year in which more than a million people entered the EU by making the short sea crossing from Turkey to the Greek islands.
Speaking at the end of a two-day European Council summit in Brussels which was dominated by the migration crisis, the Prime Minister said the arrangement was "a plan to break the link between getting on a boat and getting settlement in Europe.... a plan to bust the business model of the smugglers".
Mr Cameron said: "For the first time in this crisis, I believe we have a plan that, properly and fully implemented, could make a difference."
Any migrant arriving in Greece after midnight on the evening of March 20 will be given a swift individual interview to determine whether they will be allowed to remain or sent back to Turkey. The arrangement will impose a heavy logistical burden on the Greek authorities, and Mr Cameron said the UK stands ready to send more border officials to support the processing of migrants potentially numbering in many thousands.
The EU will take in one Syrian refugee from camps in Turkey for each irregular migrant returned, in a move which is intended to break the business model of people-smugglers who have made fortunes by providing spaces in boats to desperate refugees.
In intensive talks in Brussels, Turkish PM Ahmet Davutoglu won EU agreement to accelerate visa liberalisation for its 75 million nationals, who could gain visa-free access to the Schengen borderless area - which does not include the UK - as well as the reopening of long-stalled negotiations on its application to join the EU.
Turkish concerns about the slow delivery of three billion euro (£2.3bn) promised by the EU last November were addressed by including a commitment to identify within the coming week a list of projects which will receive funding. A further three billion euro are being made available after the initial tranche of support runs out.
Mr Cameron stressed that Britain will not offer visa-free access to Turkish nationals, and said the agreement would not add to the 20,000 Syrian refugees which the UK has promised to accept over five years.
A spokesman for European Council president Donald Tusk said that the agreement made clear that any removals would have to be "in full compliance with international and EU law" and that there would be no "collective expulsions".
But the deal was described as "a dark day for humanity" by human rights group Amnesty International, whose UK director Kate Allen said: "It's absolutely shameful to see leaders seeking to abandon their legal obligations. Forcing refugees back into the hands of the very smugglers they just came from so they can have another go at exploiting them is obviously a madness. There's no way anyone should herald this as a solution."