The Prime Minister threatened new anti-union laws yesterday as Unite leader Len McCluskey urged Labour to offer voters a real alternative at the next election.

David Cameron delighted Tory MPs at question time by promising to look at changes in the law to reduce strikes such as the recent London Underground stoppage.

He sneered that Labour was going “on a cosy weekend with their trade union masters” at this Saturday’s special conference on the trade union link.

A Tory spokesman confirmed yesterday that the party was looking at “a whole range of measures” on trade unions as it prepared its 2015 general election manifesto.

Harrow East Conservative MP Bob Blackman sparked off the PM’s outburst by demanding a new threshold in trade union ballots to stop “pointless strikes” like the underground stoppage which “inconvenienced” millions of Londoners.

In a TV interview, Unite general secretary Len McCluskey urged Labour leader Ed Miliband to reject a coalition with the Lib Dems at the next election.

If Labour became the biggest party, but without an outright majority, then Mr Miliband should have “the courage of his convictions” and should govern in minority, said Mr McCluskey.

“And he should challenge those coalition parties to bring him down if necessary and go back to the people so that there’s a stark alternative.”

Asked what it was like to be seen as an “ogre” in the right-wing media, Mr McCluskey replied: “Well it’s not very nice … sometimes I just have to wear it as a badge of honour.”

Asked if he was upset by the attacks he quipped that “obviously I’m a sensitive kind of guy.”

Then he added: “If you’re being attacked by your enemies, then you can scratch your head and think well maybe I’m doing the right thing.”

David Cameron’s proposed “inquiry” into trade union practices following the Grangemouth dispute has been heavily delayed by a squabble between Lib Dem Business Secretary Vince Cable and Tory Cabinet Office Minister Francis Maude.

Three months after Mr Cameron first trumpeted the inquiry, the full terms of reference have yet to be announced.