The Charity Tax Group is asking charities for help in a campaign to prevent the removal of EU social exemptions, which it believes are worth hundreds of millions of pounds a year to UK charities.
The exemptions, which provide VAT relief for organisations working in areas such as health and education, were agreed at the time an EU-wide VAT system was introduced in the 1970s.
But in proposals published last year the European Commission said it would examine whether there was still any justification for the exemptions.
It has asked the think tank Copenhagen Economics to produce a report into VAT exemptions.
Helen Donoghue, director of the CTG, said: "Copenhagen Economics has asked us to come up with aggregated data for the scope of the social exemptions, or to produce a series of case studies about what the impact will be if they are removed.
"We need some really strong examples from those organisations providing exempt services."
Donoghue said it was likely that it would take a long time for any change to come into effect but that, if it did, it would introduce an enormous cost for charities.
"It may be something that won’t happen for five or 10 years, but they are thinking about it now," she said. "We really desperately need those who can help to come forward."