The Home Secretary followed Number 10 in slapping down Adam Afriyie’s plan to table an amendment to legislation, which paves the way for a promised vote in 2017.
Windsor MP Mr Afriyie – once the subject of leadership bid speculation – said the public was “not convinced” Prime Minister David Cameron would stick to his pledge of a vote if the Conservatives won the general election in 2015.
He said delaying posed “significant dangers”, including building support for the UK Independence Party (Ukip), and claimed there was support from “many MPs from across all the main parties” for an early vote.
But with Downing Street insisting Mr Afriyie’s amendment would not be allowed to pass “in any circumstances”, Ms May said she thought Mr Afriyie had “got it wrong”.
She added it could pose a threat to James Wharton’s government-backed private members’ bill, which aims for a referendum in 2017 to give the UK time to renegotiate the terms of its EU membership.
Ms May said: “I think what is crucial is that we have at the next election a Conservative Party that will be offering a renegotiation, a new settlement with Europe, looking to the future and then putting that to the British people in an in-or-out referendum.
“What the amendment could do, as James Wharton himself who put in the referendum bill through parliament has said, is it could actually jeopardise that bill.”
A Downing Street spokesman said of Mr Afriyie’s plan: “The PM will not let it stand.”
Tory MP Mr Wharton said Mr Afriyie’s move would delay and even “kill” his private member’s bill altogether.
Ed Miliband and Nick Clegg have dismissed Mr Wharton’s bill as a stunt designed to shore up the Prime Minister’s position with his rank and file – pointing out that it has virtually no chance of becoming law.
In May, 115 Conservative MPs backed a rebel amendment to the Queen’s Speech, criticising the failure to include a referendum bill in the government’s legislative programme.
Mr Cameron said that was impossible, because of the coalition with the pro-Europe Lib Dems, but has thrown his weight behind Mr Wharton’s bill.
Mr Afriyie said he would table an amendment bringing forward the referendum date to 23 October, 2014. “It’s in our national interest to resolve this issue as soon as possible to create the certainty and stability our country needs for the future,” he said.
“Only by setting an early date can we kick-start EU renegotiation talks and give the British people a say on our country’s future with Europe.”
Questioning Mr Cameron’s tactics of promising a 2017 vote, he added: “Many people think delaying the vote is just a tactic to allow all the political leaders to kick the can even further down the road.”
Mr Afriyie’s amendment appeared to have little support among prominent eurosceptics. Conor Burns said he was surprised by the move when the party had finally established a rare “unity and consensus” over Europe.