Andrew Lansley, the health secretary, has been accused of "bullying" NHS staff who speak out against his NHS shakeup after a senior doctor who signed a letter criticising the proposed changes was threatened with disciplinary action.

The doctor has been told to attend a disciplinary hearing later this week by the NHS primary care trust (PCT) that employs him. It claims he breached the NHS code of conduct by airing his concerns.

In the Commons Andy Burnham, the shadow health secretary, asked Lansley if the trust's action showed it was now his "policy to threaten NHS staff with disciplinary action if they speak out about his reorganisation". He challenged the minister to reconcile his "new top-down bullying policy" with his previous strong support for NHS whistleblowers.

Prof John Ashton, county medical officer for Cumbria, received a letter from his PCT last week after he joined 22 other signatories to a letter in a national newspaper criticising Lansley's health and social care bill. The letter read: "You are bound by the NHS code of conduct and as such it is inappropriate for individuals to raise their personal concerns about the proposed government reforms." Ashton will have to "explain and account" for his actions at the hearing.

Lansley told Burnham he did not know about the case and declined to comment.

Meanwhile, Liberal Democrat activists have submitted an emergency motion on the NHS shakeup to their spring conference in a last-ditch effort to persuade the party leadership to oppose the bill. The motion calls for the entire section of the bill extending competition in the health service to be scrapped – as a minimum.

The group is trying to row behind the Lib Dem peer Lady Williams, who has called for part three of the bill – devoted to extending competition – to be ditched. But some of the activists believe the bill should be dropped altogether.

Labour will call a vote on Wednesday demanding the publication of the NHS risk register – which details what could go wrong if the bill is implemented. The Conservative MP Mark Field said it was "particularly ill-advised" for ministers to continue to refuse to publish the register. He said ministers should aim to be as transparent as possible and that he would be "very surprised" if the register was not published.

The government has so far refused to publish the document, in spite of a ruling from the information commissioner demanding that it be made public.