It is a further indication that the Tory revival engineered by leader William Hague in the spring may be losing momentum.
Meanwhile, support for Labour has topped 50% in the regular monthly Mori poll for the Times.
The poll, which excludes the 24% who did not express a preference, puts Labour on 51% - up 2% since last month - and the Tories on 29% - down 4% from July.
Tory woes
The figures come after a month in which Mr Hague was ridiculed in the press for boasting in a magazine interview that he used to drink 14 pints of beer in a day as a youngster.
Earlier in August, Tory adviser and millionaire businessman Ivan Massow defected to Labour.
The survey of 1,916 adults also comes after Chancellor Gordon Brown's multi-billion pound spending spree for public services in last month's Comprehensive Spending Review, and Labour's assertion that the Tories would cut £16bn from the CSR spending plans.
Mr Hague's personal satisfaction rating is down five points from -20 in July to -25. Even among Conservative supporters it is down from +12 to +5.
Lib Dem boost
Prime Minister Tony Blair's personal ratings rose eight points from the -8 recorded in July, and there are now the same number satisfied as dissatisfied with his performance, Mori found.
The Mori economic optimism index, measuring those who think the economy will improve over the next year minus those who think it will not, stood at -7, up three points from last month and its highest level since March.
The Liberal Democrats can take heart from the poll, which saw their ratings up three points to 15%, and leader Charles Kennedy's personal satisfaction rating increased from +12 to +14.
A Labour Party spokesman said of the latest poll findings: "It is unsurprising to see Hague support slump, as he has run away from answering Labour's key challenge this summer, namely where would the Tories' £16 billion public spending cuts guarantee fall."