Charity representatives campaigning against new powers given to HM Revenue & Customs will write to human rights organisation Liberty asking it to back their campaign to have the law repealed.
Specialist charity law firm Bates Wells & Braithwaite said it felt that the 'fit and proper persons' test for charities, introduced in the Finance Act in April, breached the human rights of charity employees.
The test gives HMRC the power to withdraw tax relief from any charity if it feels any of its senior employees or trustees is untrustworthy.
Rosamund McCarthy, a partner at Bates Wells & Braithwaite, said the law could, in effect, give HMRC the power to dismiss charity employees and trustees and bar them from working in the sector again, based on suspicion rather than proof of wrongdoing. It also gave those employees no right to appeal, she said.
The firm is working with umbrella bodies and other prominent lawyers to have UK charities exempted from the rules, which cover tax relief given to charities in the European Union.
A spokeswoman for HMRC said it assumed that all people appointed by charities were fit and proper persons, unless HMRC had other evidence.
"The fit and proper persons test helps to protect the taxpayer against sham charities and fraudsters while preserving an efficient service to charities," she said. "Nobody wants to see taxpayers' money being handed over to fraudsters, and this test helps to ensure that does not happen."
A spokeswoman for Liberty said it would review the action it would take once it had received the letter.
