The government has said a referendum is not necessary

The prime minister insisted Britain's so-called "red lines" on issues such as national security, justice and social services were secure.

Pressed by the BBC on whether there was still an "if" over holding a poll, Mr Brown replied: "Yes, of course."

The Tories say ministers could face public anger if a vote is not held.

Shadow home secretary David Davis warned on Sunday that momentum for a referendum was growing because people believed they had been promised one.

Constitution 'abandoned'

Renewing his own party's call for a poll, Mr Davis told the BBC: "People are going to say, you promised this before - this treaty is almost the same - 90% the same - as the treaty you promised a referendum for."

But Mr Brown told BBC Radio 4's Today programme on Monday: "If we succeed in achieving our red lines in all the detailed negotiations, there will, in my view and in the government's view, be no need for a referendum."

In a 10 Downing Street webchat on Monday Foreign Secretary David Miliband was forthright in his view that no referendum was needed.