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."
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The Democratic presidential candidate may also have shown his cards on his choice of running mate.
The US president also shared a post on Twitter accusing Dr Anthony Fauci of misleading the public over hydroxychloroquine.
Fears over an impending second wave of coronavirus dominates Wednesday’s front pages.