Counting was under way in local elections as polls closed with Prime Minister Gordon Brown braced for a pummelling from voters.
Labour insiders fear the party could shed up to 200 council seats amid widespread anger over the 10p tax rate abolition and sniping at Mr Brown's leadership.
The crucial London Mayoral race is on a knife edge, with incumbent Ken Livingstone struggling to fend off the challenge from Tory rival Boris Johnson.
Showers across much of England and Wales are unlikely to have eased the Prime Minister's nerves - with Conservative voters traditionally considered more likely to brave bad weather.
A poor poll showing could make life extremely difficult for Mr Brown, with backbenchers increasingly concerned about their own prospects at the next general election.
However the hammering suffered by Labour when the same council seats last came up for grabs means it is starting from a low ebb and could portray a loss of fewer than 50 seats as better than expected and claim David Cameron has yet to establish the Tories as a Government-in-waiting.
The capital's residents were choosing London Assembly members as well as a Mayor, with 4,102 councils seats being contested across the rest of England and Wales.
Mr Brown and his wife Sarah cast their votes early on Thursday morning, visiting Methodist Central Hall in Westminster, just a few minutes' walk from Downing Street.
The vote is seen as the first substantial verdict on Mr Brown's premiership, although there were by-elections immediately after he took over from Tony Blair.
One recent poll showed Labour trailing the Tories by 18 percentage points, but others have been less damning.
