David Cameron used most of his interview on the Marr show to do what he once urged his party not to do: "bang on" about Europe. But after being given free rein to outline his EU renegotiation plan, he did provide a story. Asked whether he supported the introduction of a turnout threshold for strike action, a proposal championed by Boris Johnson, he confirmed that the policy would appear in the next Conservative manifesto.

He said: "I think in these essential services, like the London Underground, the pain caused to people trying to get to work and trying to help their families by these strikes, which are often supported by a relatively small percentage...I think it’s hugely damaging and so I think the time has come for setting thresholds in strike ballots in essential services. It’s not something I can achieve in a coalition government. It’s something that will be in our manifesto."

Although Cameron did not specify a threshold, the plan would likely mean the banning of strikes when fewer than half of all trade union members vote (just 30 per cent of all RMT members took part in the most recent ballot).

It is one that Labour, unsurprisingly, opposes. Ed Miliband told the Evening Standard earlier this week: "I’m personally not convinced of the case for it. I think a better way forward is to say, look, let’s build good relations. Let me make a point on this: we have elections and we don’t have a threshold for turnout for elections. There are other ways to build good relations." Labour MPs and trade unions frequently point out that Boris Johnson wouldn't have been elected (or re-elected) London mayor under the turnout rule. Just 45 per cent voted in the 2008 mayoral election and just 38 per cent in the 2012 contest.

But while the Lib Dems have not endorsed the policy, I would not be surprised if they agreed to it as part of a second coalition agreement with the Tories. Ed Davey, for instance, a close ally of Nick Clegg, recently said that the proposal was "worth looking at". More broadly, many Lib Dems, including Clegg, generally loathe the unions due to the funding they provide to Labour.

Update: Here's how TUC general secretary Frances O'Grady has responded to Cameron's announcement.

“Workers in Britain already face the toughest barriers to taking action to defend their living standards of any almost any advanced democracy. This has helped turn our country into one of the most unequal where billionaires co-exist with food banks.

“The Prime Minister now wants to make industrial action even more difficult. This shows his readiness to take the side of employers and big business against ordinary working people as his party's backwoodsmen call the shots.

“With workers facing the biggest and longest cut in their living standards since Victorian times and growing insecurity at work, we need policies to boost incomes, create decent jobs and give people a real say at work, not this return to Thatcherite nostrums.”