Nick Clegg is facing possible defeat over the government's NHS changes at his spring party conference next weekend when a heavyweight group of Liberal Democrat figures table an amendment opposing the "damaging and unjustified market-based approach".

Evan Harris, a doctor and former MP and vice-chair of the party's ruling federal policy committee, will table the amendment, supported by the former cabinet minister Lady Williams, registering their concerns that the current legislation will lead to a widening of UK health inequalities if left unchecked.

The government is proposing to give GPs control of more than 80% of the £100bn NHS budget at the same time as driving through efficiency savings of £20bn. The package includes the abolition of primary care trusts.

Defeat at his own conference on a central plank of the government's public service reform agenda would mean Clegg would have to choose between ignoring a vote by his grassroots and negotiating concessions from the prime minister.

The amendment complains that some of the proposed changes in the health bill "have never been Liberal Democrat policy, did not feature in our manifesto or in the coalition agreement, which instead called for an end to large-scale top-down reorganisations".

The changes sought by Harris and Williams include:

• No decision about the spending of NHS funds to be made in private and without proper consultation, as can take place by the GP consortiums proposed by Andrew Lansley, the health secretary.

• Restoration of the NHS as the preferred provider, only allowing new private providers where there is no risk of "cherry-picking" that would destabilise the existing NHS. At the moment, the legislation brings an end to the NHS as preferred provider.

• NHS commissioning retained as an entirely public function, rather than the subcontracting of commissioning to private companies. At present primary care trusts are democratically accountable and open to public scrutiny.

• Continued separation of the commissioning and provision of services to prevent conflicts of interest.

• Healthcare commissioning to be carried out by locally elected health boards or local authorities, with the ability to vary a fair local tax in order to invest in local healthcare services.

Those tabling the amendment believe there are signs more concessions could be forthcoming.

In a letter circulated to canvass support, Harris cites an apparent climbdown by the government this week over price competition in the NHS to prove to Lib Dems they can secure further concessions. Writing that one demand in their amendment has already been met, they said: "The amendment calls for the ruling out of any price competition in the NHS, and only this week the government have bowed to pressure to partially accept this, so we should be encouraged to use our conference debate to demand more changes."