David Cameron has said he is determined to reform Britain’s relationship with the EU as he seeks to kick-start talks with European leaders on his plans.
At a summit in Latvia, he will outline changes he wants to see, including restrictions on benefits for migrants.
The prime minister said he expected “lots of ups and downs” but was focused on giving people a “proper choice” in a referendum due to be held by 2017.
EU leaders are in Riga to discuss relations with ex-Soviet states.
Mr Cameron said this issue was very important in its own right but that the gathering was also “an opportunity to start some of the discussions about reform of the EU”.
“All I will say is that there will be ups and downs,” he told reporters as he arrived at the conference.
“You will hear one day that ‘this is possible’; the next day something is impossible.
“But one thing through all this will be constant and this is my determination to deliver for the British people a reform of the EU so they get a proper choice in that referendum we will hold before the end of 2017, that will be constant.”
The prime minister is meeting his European counterparts for the first time since securing his re-election and a majority Conservative government – at an event that will be dominated by the EU’s relationship with Russia and its neighbours.
Legislation paving the way for the UK referendum – which was promised in the Tories’ election manifesto – is expected to be published next Thursday, the day after the Queen opens Parliament.
BBC deputy political editor James Landale said Mr Cameron would use the summit to sound out other European leaders about what they might be willing to accept before the start of “intricate” negotiations in the coming months.
While other EU leaders, he added, were “scared witless” by the possibility of the UK leaving the EU and we were willing to listen and consider a deal, there were some things – such as changes to freedom of movement rules – which were “sacrosanct”.
Mr Cameron has not revealed the full details of what he is seeking from any changes, but he is expected to demand an opt-out from one its core principles of forging an “ever-closer union” between member states.
He will also try to get more powers to block or opt out of new EU laws, and for restrictions on welfare benefits for migrants until they have lived in the UK for four years.
In a speech on Thursday, the prime minister said welfare changes would be an “absolute requirement in the renegotiation”.
Mr Cameron has said he wants the UK to stay in a reformed EU but has so far refused to say whether he would start calling for Britain to leave if he does not get what he wants.
Finnish Prime Minister Alexander Stubb said he was “hopeful” a deal could be reached which would be satisfactory to the British people.
“Finland is very much in favour of the pro-reform agenda that David Cameron drives. We believe that there are issues that can be discussed with Britain,” he said. “There is no point in putting up any barriers at this particular stage and we’re all ears.”
Ireland’s Europe Minister Dara Murphy said he was “optimistic” the UK would remain within the EU.
“Much of what has been suggested and proposed to date by David Cameron would be to the advantage of all of the people of the European Union, particularly in the space of regulation,” he said.
But Estonian prime minister Taavi Roivas warned he would oppose any attempt to roll back the free movement of labour in the EU.
“I wouldn’t scroll back the basic freedoms but saying no to all ideas of change would be wrong as well.”
This week, a number of UK business leaders started to highlight the potential implications of the referendum, with the president of the CBI said businesses should “speak out early” in favour of remaining in a reformed EU,
The Labour Party, SNP, Plaid Cymru and the Lib Dems are in favour of staying in the EU. UKIP, which got almost four million votes but only one MP in the election, want to leave.