Nick Clegg's problems with the NHS reforms have dramatically resurfaced as his party's grassroots threaten to force MPs to vote against the government unless their demands for changes to the bill were met in full.

The deputy prime minister and the prime minister, David Cameron, promised on Wednesday to "pause and listen" to growing concerns over the scale of reforms in the health bill.

But in an escalation of the controversy that now threatens to destabilise the coalition, Liberal Democrat activists demanded that amendments championed by Shirley Williams and endorsed by the party's spring conference be taken on in full.

In an unprecedented show of force about 1,200 Liberal Democrat party members, councillors, former parliamentary candidates, peers, conference representatives and council leaders instructed their leadership to "fully respect the declared view of the party".

Organisers of the Liberal Democrat revolt also condemned efforts by the government to "spin" their way out of the row. Since announcing a "pause" to the progress of the health bill, the government has suggested that some amendments would be tabled to clarify that GPs who do not want to take charge of the NHS budget for their patients would be able to leave it to other GPs in their area.

The ability of private firms to "cherry pick" the most lucrative NHS services would also be restricted and a regulator will be told to prioritise value for money, rather than promote market competition, it emerged.

However the Liberal Democrat grassroots made it clear that they want the government to go further and declared they will pressure their MPs to block the bill unless they are satisfied by future changes.

Dr Evan Harris, the party's former health spokesman, said: "The Liberal Democrats have already compromised on health policy in making the Coalition Agreement and we are not prepared to see our parliamentarians being put under pressure to vote for market reforms and other changes that go beyond the terms of the coalition agreement and the motion that we passed."

The Liberal Democrats voted almost unanimously at the party's conference in Sheffield last month to give councillors a central role in GP commissioning and in scrutinising foundation trusts.

They called for a ban on cherry-picking by private companies and declared there should be a continued separation of commissioning and provision of services to prevent conflicts of interest.

Faced with certain defeat, the party's leadership promised to take the critical messages back to government.

A government spokesman said: "We are taking the opportunity of a natural break in the passage of the bill to pause, listen, reflect and improve. This will help realise our ultimate goal of modernising the NHS to protect it for the future."