David Cameron wants to deploy Royal Navy vessels close to the coast of Libya to deter thousands of migrants from embarking on the perilous sea journey to Europe this summer.
The Prime Minister told European Union leaders at a summit in Brussels that he is extending the deployment of HMS Enterprise on anti-trafficking operations in the central Mediterranean at least until the summer, and wants to see the mission expanded into Libyan territorial waters to enhance its deterrent effect.
UK officials said that people-smuggling boats could be turned back to the Libyan shore for destruction, if the necessary co-operation can be established with local coastguards. And they said Britain would also like to see closer Nato co-operation with Turkish coastguards in the Aegean Sea so that more intercepted boats can be sent back to Turkey without ever arriving in the Greek islands.
Mr Cameron and the 27 other EU leaders hope to seal an agreement with Turkish PM Ahmet Davutoglu on Friday which would see migrants returned to Turkey if they fail fast-track asylum interviews and appeals after arrival in Greece.
In return, the EU would accept one Syrian refugee from camps in Turkey for every irregular migrant returned, up to a limit of 72,000. Ankara stands to see a 3 billion euro aid package doubled to 6 billion euro (£4.7 billion) and could gain visa-free access for its 75 million nationals to the EU's Schengen border-free zone and the revival of talks on eventual accession.
Mr Cameron insisted that the proposed deal will not add to the 20,000 vulnerable Syrian migrants Britain has already agreed to accept from camps in the region over five years. And he said the UK's "special status" outside the Schengen area meant there would be no visa-free travel to Britain for Turks.
But he made clear he backs moves to return migrants, saying: "What matters today is actually busting the business model of those people-smugglers and therefore breaking the link between getting on a boat and getting settlement in Europe. If we can get an agreement that returns the migrants from the Greek islands to Turkey that would be good progress."
In talks over dinner in Brussels, Mr Cameron said he was sending several more officers from the National Crime Agency to assist with border operations in Greece and Turkey.
But he warned fellow leaders they must not "take their eyes off the ball" of Libya because of their focus on the crisis in Greece, where more than 800,000 people arrived by sea in 2015 and 125,000 in the first two months of this year.
If the new deal with Turkey succeeds in cutting migration in the eastern Mediterranean, economic migrants may divert to the north African country as improving weather makes the crossing to Italy more viable during the summer months, he said. Some 170,000 migrants arrived in Italy from Libya in 2014 and 150,000 last year, but the death toll was much higher than on the Turkish route because of the longer distances involved.
Mr Cameron said that the EU should be opening discussions with the newly-established Libyan government of national accord under prime minister-designate Fayez al-Sarraj on the extension of its current Operation Sophia mission to within several miles of the Libyan shore.
The mission is currently able to operate only in international waters, but has already detained 53 smugglers and destroyed 90 vessels - including 69 rubber dinghys, 20 wooden boats and one fishing vessel - since last July.
Some people-smugglers have been taking advantage of the fact that Operation Sophia vessels cannot sail close to the Libyan coastline by taking boatloads of migrants to the edge of international waters before casting them loose.
As well as signalling his readiness to keep HMS Enterprise in the area if the Operation is extended beyond July, Mr Cameron said Britain was willing to deploy other assets, which could include Border Force cutters and Wildcat or Lynx helicopters, said officials.
He believes that an EU naval presence close to the Libyan shore would deter many migrants from setting out and would allow local coastguards to escort other boats back to shore, where they can be destroyed.
The proposed deal with Turkey was denounced by the Refugee Council as "immoral, unworkable and probably illegal". Chief executive Maurice Wren said: "The fact that the Government is boasting of its intention not to lift a finger to help more refugees find safety in the UK is emblematic of its lack of moral leadership and political courage to do the right thing."
And Amnesty International refugee director Steve Symonds said: "The plan lacks both logic and compassion. By abandoning their legal obligations, European leaders won't put a stop to refugee migration."
But Mr Cameron's official spokeswoman said the UK believes the proposed new arrangements with Turkey can be imposed "in line with EU and international law" as individual cases would be assessed on their merits.
And European Commission president Jean-Claude Juncker insisted that any arrangements would respect both European law and the Geneva Conventions.
Dutch Prime Minister Mark Rutte suggested that the prospect of immediate return from the Greek islands to Turkey would halt the flow of migrants by the eastern Mediterranean route "in three to four weeks" after finalisation of the deal.
Meanwhile, EU auditors issued a harsh verdict on efforts to control migration from outside the bloc prior to the current crisis. External migration operations on southern and eastern borders between 2007 and 2013 lacked a clear strategy and suffered from "complex governance, insufficient coordination and the absence of a funding overview", said the European Court of Auditors.