The Conservatives must give voters "greater certainty" that they will be given a say on Britain's membership of the European Union, Home Secretary Theresa May acknowledged.
After the Tories suffered heavy losses to the UK Independence Party in the council elections, May said it was essential voters believed the party would honour its promise to hold an in/out referendum on EU membership after the next general election.
However she rejected calls from some senior Conservatives to bring forward the date of the referendum and stage it in this parliament in order to finally kill off the appeal of Ukip to Tory voters.
She said that it was important that they stood by David Cameron's plan to re-negotiate the terms of Britain's membership and then put the new settlement to the country.
"If you want to take a re-negotiated settlement to the British people, you have got to re-negotiate it," she told the BBC Radio 4 Today.
"We can look at whether we can give some greater certainty in terms of the referendum. I think what we need to do is to be able to show people that we will hold that referendum, we will hold that referendum after the general election."
She did not rule out introducing legislation in the current parliament to pave the way for a referendum after the next general election - despite the tensions it would create with their Liberal Democrat coalition partners.
"I am not saying that's definitely what we are going to be doing," she said.
For Labour, the shadow leader of the Commons Angela Eagle said it was important that the surge in support for Ukip did not allow the Conservatives to "drag the country to the right".
"It is quite clear that the vote on the right in British politics is split down the middle," she told the Today programme.
"We mustn't let them drag the country to the right and not act in the best interests of the country because they are having a spat in the Conservative family."
She insisted that Labour had made "good progress" in its target seats it needed to win in southern England, despite failing to make the sort of gains analysts believe it needed to make if Ed Mililand was to enter No 10.