Switzerland has blocked access to government archives relating to the country's ties with apartheid-era South Africa.
The head of the Swiss research programme into apartheid-era links says the move will limit his ability to properly probe Switzerland's relationship with the white regime in Pretoria.
A finance ministry statement, issued on Thursday, said the temporary decision had been taken in order to protect Swiss companies facing class action suits in the United States.
Last November, suits were filed in the US against 20 banks and multinationals from Switzerland and other countries on behalf of the victims of apartheid.
As well as Swiss banks, Credit Suisse and UBS, firms such as the oil companies Exxon and Shell, and carmakers Ford and General Motors were named in the lawsuit.
"There is the particular risk that Swiss firms, due to easier access to information, could be singled out and face exaggerated accusations," said the finance ministry in a statement.
The ministry said no other country had offered comparable open access to such information.
The archives were opened in May 2000 as part of a national research programme into relations between Switzerland and South Africa during the apartheid era.
Georg Kreis, the head of the research programme, said he wasn't surprised by the decision.
"I, of course, very much regret that the framework conditions have been changed while we are in the process of research," he told swissinfo.
"On the other hand, I do have a certain understanding that when historical research encroaches on the present then there's a risk that conditions will change and affect the conditions under which we can carry out our research," he added.
Kreis said he doubted that his team would get to the bottom of the matter as his research period was due to end in October. He expects to present his findings in 2004.
He said he had already been facing limitations - such as problems in gaining access to private files, especially from companies - and that the new restrictions would further hinder his work.
"It'll be our task to present what we have been able to find out based on limited information and statements," Kreis said.
He said that he would be making clear the conditions under which the research was conducted.
"One must particularly avoid giving the impression that we have had access to everything and that it will be a final report. We want in no way to offer a whitewash," he added.
The non-governmental organisation, the Swiss Campaign for Apartheid Caused Debt Cancellation and Reparations, criticised Switzerland's decision.
"We ask ourselves what the finance ministry has to hide from the public," the group said in a statement issued on Thursday.
There was also reaction from the political parties. The rightwing Swiss People's Party and the centre-right Radicals and Christian Democrats welcomed the move as defending the interests of Swiss firms.
The centre-left Social Democrats and the Green Party said they were shocked and would be asking parliament to help uncover the full extent of the links between Switzerland and apartheid-era South Africa.
swissinfo, Isobel Johnson