The Conservatives have widened their lead over Labour to 14 points, according to the latest "poll of polls" for The Independent.
David Cameron's party has also reached the 40 per cent support level seen as crucial for delivering a clear general election victory. Its backing has risen by three points in a month at the expense of falling ratings for smaller parties.
The weighted average of five polls last month puts the Tories on 40 per cent (up three points), Labour on 26 per cent (up two), the Liberal Democrats unchanged on 19 per cent and other parties on 15 per cent (down five). Repeated at a general election, that would give Mr Cameron an overall majority of 68 seats – a similar margin to that achieved by Tony Blair in 2005.
The figures were based on the last polls before MPs left Westminster for their summer recess, including a ComRes survey for The Independent which put the Conservatives on 42 per cent and Labour on 24 per cent. The Conservatives have now enjoyed a consistent double-digit lead over Labour since the beginning of the year. John Curtice, professor of politics at Strathclyde University, compiled the figures and said there appeared to have been a "decisive long-term shift in the public mood" since the spring last year. Last autumn's recovery in Government support now seems to have been "little more than a temporary blip", he said. "Labour evidently has a tough job on its hands to shift this now long-standing public mood once more."
Support for other parties such as the UK Independence Party and the Greens reached a peak in June amid anger over the MPs' expenses scandal.