Prime Minister David Cameron has squandered the Conservatives' lead in the polls, as voters turn against his health reforms, according to a Guardian/ICM poll.

The Tories are down by four percentage points in a single month, slipping from 40% to 36% since January.

Labour is one point ahead, on 37%, with Ed Miliband's party up from 35% last month. The Liberal Democrats have slipped back two to stand at 14%, and the combined total of the smaller parties has climbed by four points, to 13%.

As the prime minister hosted a special NHS summit, which excluded the professional bodies most opposed to his health and social bill, the public is siding with those royal medical colleges who want the legislation ditched.

An outright majority of respondents, 52%, say that the bill – which would overhaul NHS management, increase competition and give family doctors more financial responsibility – should be dropped. That is against 33% who believe it is better to stick with the plans at this stage.

The 19-point overall margin in favour of abandoning the legislation is mirrored in strong leads for killing the bill across all social classes and regions, as well as among male and female voters.