Shadow health secretary Andy Burnham said he wanted reassurances that that a Government-backed inquiry into the conduct of industrial disputes was not a "political call".
"Of course if there's been intimidation it's unacceptable and that should apply to unions as well as employers," he told BBC One's Sunday Politics show.
"I think I need reassurance that this isn't a political call by Mr Cameron designed to report near the election, so you know we'll see how this develops."
More top news
Food prices will rise across the UK unless EU workers are allowed to remain as part of the Brexit negotiations, industry bosses have warned.
A north / south split in conditions tomorrow. Remaining mild
Polluting cars and vans could be permanently blocked from the French metropolis if it keeps to its mayor's recent clean-air commitments.