MPs in both parties are unhappy with the Bill's current proposals, which they say will have no effect on the charitable status of public schools.
The support of modernising, Cameronite Tories could also be enlisted to introduce a more demanding public benefit test.
John Grogan, Labour MP for Selby, said there was "unease" among his colleagues.
"Labour backbenchers want a robust test of public benefit," he said.
Grogan said some Labour MPs wanted to copy last year's Scottish Charities Act, which denies charitable status to a fee-paying organisation if access to its services is "unduly restrictive".
He said there was also support for the concept of stripping public schools of their charitable status and awarding them tax reliefs separately.
Martin Horwood, Liberal Democrat spokesman on the Bill, said there was a "real possibility" the party would revive the less severe Lord Phillips amendment, which required the Charity Commission to consider the effects of charities' charging policies on public benefit.
He said he would seek Labour and Conservative support for an improved public benefit test.