Labour gains in the county elections are the result of UKIP splitting the right-wing vote, Crawley MP Henry Smith has argued.
The MP blamed UKIP for taking votes from Conservatives and the loss of four Tory-held seats in Crawley at West Sussex County Council.
He said the danger was UKIP supporters who want less EU involvement could inadvertently let pro-Europe Labour in by the back door by giving them a greater share of the vote.
He said: “Labour didn’t advance much at all. Where they gained seats it was where UKIP took right-of-centre votes. Had there not been UKIP candidates in those seats we would have won.
“What we saw was there’s no great enthusiasm for people to vote Labour, people still remember the appalling mess they left the economy in.
“I would urge people to think very carefully about how they vote. The danger is voting UKIP and splitting the vote could let Labour in through the back door. There would be an irony if the result was more European integration.”
The MP said UKIP’s successes - which included ten seats in West Sussex and thousands of votes across Crawley - showed public attitudes towards the EU had hardened.
He said: “There’s a clear desire from a sizable proportion of the electorate for a referendum on EU membership, which is why I’ve gone against three-line whips and the Prime Minister has had me in his office trying to persuade me to vote with government.
“We want it sooner rather than later - this year. I think we should start laying the legislation with or without the Lib Dems.
“Fifteen years ago people thought leaving the EU was mad, now it’s mainstream political thought.”
Mr Smith said the mid-term result did not necessarily have a bearing on the General Election.
He added: “By no means do I think the election is in the bag, it’s something that will have to be fought for. I look forward to debating policies with opponents and hearing how Labour would pay for deficit, and what UKIP’s policies are on schools and health.”