Britain can be treated as "the special one" in Europe and efforts made to accommodate its desire to have a different relationship with the European Union, the EU commission president said on Thursday in a keynote speech that will please the prime minister.

José Manuel Barroso said: "I passionately believe that Europe is stronger with the UK as its member, and that the UK is stronger as a member of the European Union than on its own. But I do acknowledge that for historical, geopolitical and economic reasons the case of the UK may be seen as a special one."

But he insisted that any exemptions or opt-outs for the UK could not be transformed into exceptions for anyone else.

"We can, and should, find ways to cater to the UK's specificity, inasmuch as this does not threaten the union's overall coherence," Barroso said.

The remarks suggest Barroso believes the EU might be able to offer Britain concessions in any negotiations about the future UK-EU relationship after the election and before a referendum. At the same time he urged Britain to be realistic about what it can achieve, pointing out that "member states hope or pretend Europe will eventually be a bigger version of themselves – but that will never be the case."

Barroso also criticised the Conservative party, and other mainstream centrist parties, for failing to counter euro-scepticism. He said: "Confronted with the growing voices of euroscepticism and even europhobia, some mainstream political forces have internalised populist arguments rather than countering them. From the centre-left to the centre-right, political forces and actors must leave their comfort zone, I would say. Instead of abandoning the debate to the extremes, they have to recover the initiative."

But shadow foreign secretary Douglas Alexander said with days to go to the European elections, the British public were none the wiser on what specific reforms the prime minister actually wants, whether he can deliver them and what he will do if he doesn't.

He said: "You do not need a crystal ball to predict the consequences of such a state of affairs; you only need to read the history of John Major's government: an ungovernable Conservative party unable to govern in the national interest which threatens to inflict huge uncertainty on business and undermine Britain's influence abroad."

He will repeat the promise to hold an in-out referendum on Europe in the unlikely event powers are to be handed to the EU. Alexander also acknowledged that Ukip is "tapping into a deep sense of discontent".

"Millions of families feel they work harder and harder but feel themselves slipping further and further behind. There are legitimate concerns about the impact of immigration which my own party ignored for too long. And there is frustration with politics as a whole."

But he will insisted that "Labour will not follow the Conservative party's approach of first ignoring, then insulting, and then imitating Ukip.

"Where Labour differs from the Conservatives is that we know our approach must not to try and be a better Ukip, but to be the Labour party at its best.

"The truth is that the voters only know one thing about Ukip. And the more they know, I think the less they will like.

"Nigel Farage likes to say he is the only politician 'keeping the flame of Thatcherism alive'. Ukip's policies towards working people are more Thatcherite than Lady Thatcher herself."

Nick Clegg will also use a speech in London on Friday to say his support for the EU does not preclude the need for reform. He will say the Liberal Democrats understand that you cannot "secure a new settlement for Britain through a one off negotiation conducted under the threat of exit." Instead, "you do it by engaging with our neighbours, forging alliances with like-minded states and winning the argument."

He will claim only his party is united on Europe. Whereas, "caught between their backbenchers' demands and the needs of British business, the Conservative leadership has spent the last three years ducking and weaving, looking for a way out."