David Cameron is holding talks with the German Chancellor Angela Merkel in Berlin today - someone he needs to win over if he is to have any chance of pushing his EU reform plans through.
He is also due to meet the Polish Prime Minister today, as he wraps up his bid to renegotiate the terms of the UK's membership of the European Union with key leaders.
The Government will not win its referendum on Britain's continued membership of the European Union unless there is "substantial" reform in Brussels, Foreign Secretary Philip Hammond has warned.
Speaking to the BBC, Hammond said: "The Prime Minister is very clear in dealing with European counterparts that if we are not able to deliver on these big areas of concern that the British people have, we will not win the referendum when it comes.
"We expect our European Union partners to engage with us in delivering a package that will enable the British people to decide that Britain's future is best delivered inside the European Union.
"We expect that some of our partners will adopt a hard line at the start of the negotiations - that's how negotiation works - but we are very confident that, over the course of the summer and perhaps onwards through the winter, we will be able to negotiate a substantial package of reform which will address the concerns that the British people have."
David Cameron has begun a whirlwind tour of Europe to discuss his terms for renegotiating the UK's membership of the EU with key leaders.
Prime Minister plans a whistle-stop charm offensive with EU leaders this week.
Details of the In/Out referendum will be presented to Parliament in a Bill this morning.