Labour's lead over the Conservatives has halved in the last month, according to a new poll.
The YouGov survey for The Sunday Times found support for Labour had fallen from 42% to 39%, while backing for the Tories rose from 32% to 34%.
While Labour retains a five-point lead, such volatility is likely to put paid to the prospect of Gordon Brown calling a snap General Election this autumn.
There is small comfort for Sir Menzies Campbell as the Liberal Democrats gather for their annual conference in Brighton, support for his party rising from 14% to 15%.
But that is well below the levels of support enjoyed by the party under his predecessor as leader, Charles Kennedy.
The Lib Dems had a 23% vote share at the 2005 General Election.
The Prime Minister also faces renewed pressure to allow a referendum on the controversial European Union treaty.
Some 60% of voters want a plebiscite, according to the poll, with 40% believing that the treaty is the same as the ditched constitution in all but name.
:: YouGov interviewed a representative sample of 1,842 adults, online, across Britain, on September 13 and 14.
