The government is going to have to be "extremely tough" on the unions to avert a wave of strikes by public sector workers, David Cameron said.
Council workers have voted in favour of strike action in a dispute over pay, the union Unison has announced.
He said Labour was "so reliant" on unions for funding that they felt they had a "stranglehold" over the party and felt they could "dictate terms".
The strike ballot came after hundreds of thousands of members rejected a 2.45% pay offer.
Mr Cameron said he backed tough action as strikes "rarely achieve their goal".
Alistair Darling said he wanted people from the "boardroom to the shop floor" to take pay rises "consistent" with the government's 2% inflation target.
The Conservative leader spoke as the result of a strike ballot among council workers was due on Monday.
David Cameron said the government is going to have to be "extremely tough" on unions to avert a wave of strikes.
Members of Unison have been voting on whether to back industrial action after rejecting a 2.45% pay offer.
About 800,000 council workers in Wales, England and N Ireland are involved.
They include school dinner ladies, classroom assistants, architects and refuse collectors.
'Sustained campaign'
Pressed on the issue at his monthly news conference, Mr Cameron said: "I hope there won't be a series of strikes.
"I think the government is going to have to be extremely tough about this to make sure we don't have a wave of public strikes."
The offer is simply not enough to help our members cope with the huge rises in the cost of food, fuel and housing Heather WakefieldUnison
Chancellor Alistair Darling said on Sunday that he wanted people from the "boardroom to the shopfloor" to take pay rises "consistent" with the government's 2% inflation target.
Heather Wakefield, Unison's head of local government, argued that this year's pay negotiations had been "dogged" by government efforts to limit public sector pay rises to 2%.
"The offer is simply not enough to help our members cope with the huge rises in the cost of food, fuel and housing," she said.
Seven days notice of strike action will have to be given if there is a "yes" vote.
Industrial action is expected to involve strikes of more than two days as part of a "sustained campaign".
Employers have warned that jobs and services will be affected if the 2.45% offer is increased.
Unison members in Scotland are also being balloted on strike action in a separate dispute over pay.
