Strikes on the London Underground could become harder for unions to enforce under a future Conservative government, according to Prime Minister David Cameron.

Mr Cameron has pledged to introduce a threshold on union ballots for strike action within ‘essential services’, such as the Underground, should the Conservatives win a majority in next year’s general election.

Mr Cameron told the BBC he was unable to deliver the changes in coalition because of the Liberal Democrats’ opposition to the plans.

In an interview with BBC London 94.9 Mr Cameron said "I want a Conservative government to pass new legislation so that strikes in central public services can't go ahead unless there is a proper threshold crossed in terms of the number of people taking part in the ballot.

"Of course there is a right to strike in this country, but in essential services, isn't it worth saying there ought to be a threshold before a strike is called, which causes so much damage?

Mayor of London Boris Johnson has called for strikes to be unlawful unless 50 per cent of staff in a workplace take part in a ballot.

Under the current law, a strike can take place if it is backed by a simple majority of those balloted.