David Cameron will not win a general election outright and will struggle to form a government, according to Lord Heseltine.
The party grandee stunned activists by claiming that history was against Mr Cameron’s hopes of securing an overall majority.
Instead, the former deputy prime minister said he would "put money" on a hung parliament, with the Tories the largest party. Mr Cameron would then be forced to call a second election later this year to seek a proper mandate to govern.
His comments, at a meeting of party members in west London, reflect a growing anxiety among the leadership about the size of the task they still face to secure outright victory.
The results of a YouGov poll show the Tory lead narrowing to six points. A separate poll shows Mr Cameron’s personal lead over Gordon Brown has been halved in the past six months.
Yesterday, the Prime Minister put Labour on an election footing with a combative speech urging voters to give him "a second look".
He was confident that his party would narrow the gap in the polls as MPs hit the doorsteps in anticipation of a May 6 election.
Lord Heseltine, who is Mr Cameron’s adviser on cities, said at a fund-raising event last Tuesday that he did not think that the party’s lead would grow enough between now and polling day to give it a working majority.
Activists at the event, which was attended by senior Tories including Francis Maude, the shadow minister for the Cabinet Office, expressed surprise at the pessimistic message. Asked to elaborate on his comments, Lord Heseltine told The Sunday Telegraph: "The Tories are running around 10 points ahead in the polls, which should give you a very big overall majority in most of the post-war world, but at this moment it gives you an overall majority of one. "Once you know that, you know the Tories must adopt a totally determined view — there’s no room for complacency, it’s a hell of a battle. "You need a swing for which there is only one sensible post-war precedent," he said, referring to Labour’s win in 1997 when a 10.3 per cent swing earned a landslide 179 majority. "Tony Blair was further ahead in the polls in 97," said Lord Heseltine. "If I was a betting man, my money would be on the election resulting in a hung parliament with David Cameron as Prime Minister."
Asked if Mr Cameron would be able to govern, he said: "I think you won’t see a full parliament, that’s for sure. There are a range of people who are supporting the Labour Party who would never have supported it over the 1960s or 70s. People think I’m being cautious, but that’s not a bad place to be. "
A Tory spokesman said: "We are determined to win this election with a clear majority but Lord Heseltine is right that the swing needed to do so is significant.
"David Cameron has repeatedly said that we need to gain 117 seats to win a majority and that hasn’t happened since 1931. That’s why we are doing everything we can to set out the choice ahead of the election."
Lord Heseltine’s caution would appear to be borne out by research by PoliticsHome.com, which shows Mr Cameron’s personal ratings falling at an increasing rate.
When voters were asked in September whether they thought he was doing a good or a bad job, Mr Cameron scored a positive performance rating of 36. That rating fell steadily until it was 12 by the second week of February. By contrast, Mr Brown’s performance rating was minus 55 in September last year and climbed to minus 33 by mid-February.
If the trend continued, Mr Cameron and Mr Brown would both have performance ratings of just below zero on May 6, the most likely date of a general election.
There is further bad news for the Tories today with the YouGov poll showing the party down one to 39 per cent and Labour up two to 33, the smallest gap since December 2008. The Liberal Democrats are on 17 per cent.