In an angry attack on “completely out of touch” ministers, Ms O’Grady warned there were plans to toughen up anti-union legislation if privileged Prime Minister David Cameron wins an overall majority.

“There are clearly people working on detailed proposals that we haven’t seen yet,” she said in an interview.

Speaking ahead of the TUC’s annual congress, which begins on Sunday, Ms O’Grady slammed Mr Cameron for acting “cavalier” over NHS pay.

“Unless the government comes to the negotiation table, we will see a mix of industrial action and protests,” she warned.

The Conservatives have already announced plans to introduce thresholds for union strike ballots.

TUC delegates will discuss the plans next week.