Under proposals to make underground an 'essential service', minimum operation would have to be provided by tube workers

David Cameron could promise to class the London Underground as an "essential service" as part of the next Conservative manifesto in an effort to stop strike action disrupting the tube.

As tube workers began a 48-hour strike over job losses caused by ticket office closures, it emerged that the Tories were looking very closely at new restrictions on strike action, although it would not be done under the coalition.

Under the proposal, a minimum service would have to be provided by London Underground workers, like current rules around fire service and police strikes.

It comes as the Conservatives also consider new thresholds to make sure a majority of union members vote for strikes, rather than just a majority of those who cast their ballots.

A Conservative source said: "It's right that we look at issues like ballot thresholds and minimum service agreements in order to protect passengers on vital public transport networks. We will consider these and other reforms for our manifesto."

Bob Crow, the head of the RMT union, condemned No 10 for "playing politics".

"Rather than threatening to declare martial law in London as part of his ongoing battle with Boris Johnson for the future of the Tory party, David Cameron should be telling the mayor stick to his election promise to Londoners not to close ticket offices," he said.

"Playing politics with a dispute that is simply about jobs, safety and services gets us nowhere at a time when talks are the only way forwards."

Johnson, the London mayor, has made the most vocal demands for reform of strike laws.

He told BBC Radio 4's Today programme he supported new voting thresholds on industrial action ballots and argued many cities had total bans on strikes by essential transport workers.

"I think [minimum thresholds] would be reasonable for vital public transport functions such as the London Underground which has to keep the greatest city on earth moving, on which millions of people depend on for their livelihoods," he said.

"And people say: 'Oh, well, you only got elected on 40% and so on', well I quite understand that point. I just think that there's a difference between a local election or a political election and the operation of a vital public service and there are plenty of other capital cities around the world that actually have a total ban on strikes by their mass transit workers."