Volunteering organisations have accused the Government of rushing through legislation that would allow immigrants to gain fast-track citizenship by volunteering.
The Borders, Citizenship and Immigration Bill introduces an "activity condition" that allows immigrants who have been in the UK for five years to gain citizenship within one year rather than three if they can prove they have been an "active citizen" by volunteering.
But the bill does not set out what constitutes volunteering or indicate how much volunteering would be required to get citizenship faster.
Parliament is expected to pass the bill on 14 July after it goes through its report stage and its third reading at the House of Commons in one day.
Stephen Brown, senior public affairs officer at Volunteering England, told Third Sector that from an early stage the Government had wanted the legislation to be passed before Parliament's summer recess, which begins on 21 July.
"Because the report stage and the third reading will take place on the same day, it is unlikely there will be time for significant amendments," he said.
Volunteering England issued a briefing to MPs in May that warned the "strong and explicit incentive to volunteer" could "reduce volunteering to a tick-box exercise".
The briefing also warned of a "rushed implementation window" that meant organisations supporting volunteering groups faced an unexpected burden of explaining the new scheme to the groups and aplicants.
Baroness Julia Neuberger, who left her post as the Government's volunteering champion last month, told Third Sector the bill used volunteer-involving organisations as an instrument of policy, which she said was unacceptable.
But preventing the proposal from becoming law was difficult, she said. "I don't know whether we'll be able to do anything, because people just don't know about it," she said.
Dame Elisabeth Hoodless, executive director of volunteering charity CSV, welcomed the proposal but raised concerns about the scale of the plans and the need for extra funds so the sector could cope with more volunteers.
A Home Office spokeswoman said: "Our proposals for earned citizenship are designed to complement our points-based system by ensuring that those who enjoy the full benefits of citizenship have earned them."