The Prime Minister is prepared to offer £275 million over the next two years to Turkey to help it cope with the scale of the refugee crisis it faces.
The money comes on top of £200 million pledged to 2020 to help African countries address some of the problems which have driven many migrants out of their home countries.
The extra support on offer for Turkey will be set out as European Union leaders gather for a meeting after the conclusion of a major international summit on the migrant crisis in Malta.
A Number 10 source said: “We are sat in the western Mediterranean which was the focus at the start of the crisis but in recent months the focus has been on the eastern and the route from Turkey to Greece where you see a lot more of the numbers of Syrian refugees as opposed to the route where we are sat today, which is more illegal migration.
“We remain really concerned about that route and the support that we should be providing to countries in the region.”
Setting out the scale of the challenge facing the Ankara government, the source said Turkish coastguards had picked up 63,000 migrants this year.
The source added: “They have got two million refugees, it has cost them almost seven billion euro to date and we think there is much more the EU can be doing there, alongside the work we have been doing in Jordan and Lebanon.
“So we are going to say that we would be willing to provide up to £275 million over the next two years as part of a stepped-up contribution from European countries to help Turkey.”
“So we will put this on the table.”
The money could be used to fund new refugee camps and provisions for those left homeless by the conflict in Syria.
The UK money would also help Syrian refugees stay closer to their homeland, which has been torn apart by the bloody civil war and the rise of Islamic State.
“The more you contain the problem in and around Syria so that if and when you find a solution on Syria it’s easier for people to go home,” the source said.
The funding could also be used to tackle the “small-scale industry” selling rafts to desperate migrants trying to leave Turkey for Europe.
The money for Turkey and Africa would come out of the 0.7% of gross national income committed to overseas aid.
The Number 10 source said: “It is us looking at how we focus our spending on humanitarian assistance, but also dealing with an issue that has repercussions for us in Britain. So we are investing our aid money upstream and overseas to better manage the problem arriving at our shores.”
As well as the extra funding for African nations, the Government will also urge them to do more to accept the return of migrants who have tried to enter the European Union illegally.
“The Prime Minister will be emphasising to the Africans that they have got to work with us on returns. We welcome the co-operation they have shown so far but it’s very important that we develop the situation where we are able to return illegal migrants who arrive in the Mediterranean,” the source said.
The extra support for Africa and Turkey came after Mr Cameron vowed the UK will play a “huge and historic role” in helping to tackle the migration crisis, including stepping up efforts to “smash” gangs of human traffickers.
The Prime Minister said it was “the biggest problem facing Europe today”, with a movement of people greater than any seen since the end of the Second World War.