Voluntary sector umbrella groups will survey their members this week to find out how many stand to lose money in collapsed Icelandic banks.
The survey comes after a meeting last week between sector leaders and Treasury minister Paul Myners, who asked for more details on the scale of the problem.
The estimated amount charities have lost varies widely. However, charity leaders who met Myners have said the sector could have lost as much as £120m.
The survey of members will be conducted by the NCVO, Acevo, the Charity Finance Directors' Group and the Charities Aid Foundation. A standard format will be used and anonymity will be guaranteed.
The purpose of the survey will be to find out the extent of charities' losses, the type of funds their money was kept in, what percentage of reserves the potential losses represent and whether the losses affect the solvency of charities.
"It will have to be quite quick, with results within a week," said Stephen Bubb, head of chief executives body Acevo. "We need people to come clean, not least because it's in their interest to do so.
"I wouldn't be surprised if there are quite a lot of charities that have lost relatively small sums of about £50,000. I want to make sure that charities are at the top of the list for compensation, rather than local authorities."
Keith Hickey,?chief executive of the CFDG, said: "I would now urge all charities that have been affected to come forward immediately so that we can assess the extent of the problem and work with the Office of the Third Sector, the Foreign Office, the Ministry of Justice and the Treasury to do all that we can to recover charities' funds tied up in Icelandic banks."