Former chancellor Lord Lawson has announced he will lead a Conservative campaign to leave the EU.
The Eurosceptic peer said he has taken the role as president of the Conservatives for Britain and will lead a cross-party exit movement ahead of the EU referendum, due to take place by the end of next year.
Writing in The Times, he warned that David Cameron will only secure "wafer thin" reforms of the EU.
Lord Lawson said it is time for the Prime Minister and Chancellor George Osborne to set out "red lines" in the renegotiation of the UK's membership.
"A number of my colleagues in the Conservative party are waiting to see what the Prime Minister negotiates before deciding which way they will vote or whether they will campaign for 'in' or 'out'," he wrote.
"We cannot afford to wait that long.
"If we leave the playing field vacant, less moderate, xenophobic voices will dominate the debate and we will fail as soon as the Government, the major political parties, the CBI and trade unions declare they are backing the 'in' campaign."
He added: "If we were able to secure those reforms I would be delighted. But if we are not I will certainly vote to leave.
"Sadly, the latter is far more likely..."
Lord Lawson's intervention comes amid discussions involving Mr Cameron, Mr Osborne and Foreign Secretary Philip Hammond to renegotiate Britain's relationship with the EU.