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
Data-selling companies trade personal addresses and phone numbers for as little as 4p per record, a watchdog has warned.
A bitterly cold airmass covers the UK this weekend.
The emotion of Donald Trump's inauguration was too much for Jessica Starr, who told ITV News: "I'm so sorry... this is not America."