David Cameron will take his European Union reform bid to Spain and Portugal this week as he seeks to build support for a new settlement.
The Prime Minister will hold talks with his counterparts in Madrid and Lisbon on Friday, Downing Street confirmed.
A renegotiated relationship will be put to British voters by the end of 2017 and formal talks have begun at an official level in Brussels.
The latest round of premier-level negotiations comes amid tension among EU member states over how to deal with the growing refugee crisis.
Measures to stem EU migration to the UK - such as a four-year ban on benefits for new migrants, the removal of the requirement for "ever-closer union" in Europe, a greater role for national parliaments, moves to improve economic competitiveness and measures to protect the interests of EU states, like Britain, which do not use the euro - are his central demands.
Cutting the numbers of EU nationals migrating to the UK is one of the PM's four priorities for the renegotiation - with the focus on tightening welfare provisions to reduce the "pull factors" making the UK a preferred destination.
Others are strengthening the single market, opting out of the move towards "ever closer union" and protecting the interests of member states outside the eurozone, including the UK.
A Number 10 spokeswoman said: "The Prime Minister has been clear on the four areas where the UK is seeking reform of the European Union.
"Technical talks on these issues are now under way and while these talks take place, he will continue to hold discussions with his counterparts – including in Portugal and Spain this Friday – in order to secure reforms that address the concerns of the British people."
He will meet first with Portuguese Prime Minister Pedro Passos Coehlo before talks with Spanish PM Mariano Rajoy.
Chancellor George Osborne last week took the push to potential allies in Sweden, Denmark and Finland.