George Foulkes, a former minister in the Scottish Office, said he "will pursue any opportunity" to end the tax benefits fee-paying schools enjoy from being charities, and create a level playing field with the state sector."
This could take the form of a private members' bill or amendments to the Charities Bill. The move might well end the cross-party consensus over the content of charity law reform.
Foulkes told Third Sector that he had support from a large number of Labour MPs and especially from grass-roots members of the Labour Party for the change.
The draft Charities Bill, promised by the Government in last week's Queen's Speech, will be open to amendment during an eight-to-12 week pre-legislative scrutiny session by a Parliamentary committee. The committee will take evidence from pressure groups before the Bill is presented.
The Government has signalled its intention to compel fee-paying schools to demonstrate some kind of public benefit, but can be expected to oppose the Foulkes amendment.
If the draft Bill gets bogged down in controversy, it could prolong an already very tight Parliamentary timetable and threaten the Bill's chances of becoming law before the next election, which is widely expected to be in early 2005.
Simon Hebditch, director of policy at CAF, said there was a very real danger the Bill could be curtailed by the calling of the election.
"It's all going to depend on whether most of the hard work is done in the pre-legislative stage," he said. "If you do 99 per cent of the work there, then it can go through Parliament fairly quickly. But if there are substantial issues still unresolved then it might not make it."
He added that the Government was nervous about the issue of private schools and might not have the stomach for a fight in the last Parliamentary session before a general election.
Another possible source of controversy during the passage of the Bill is the regulation of fundraising. Despite recent press criticism of face-to-face fundraising, the Government says it is sticking by its commitment to self-regulation, with direct intervention by the Home Secretary very much a last resort.
Hebditch said it was important that the sector produced a "coherent and acceptable" system of self-regulation through the Buse Commission by next summer, when pre-legislative scrutiny would be in full swing. "If we can't, we could have a system imposed on us," he said.