Datacell, an IT firm that hosts the Wikileaks donations web page, has said it will sue Visa Europe and Mastercard for lost revenues after they refused to transact with the site.

The credit card companies this week said they would no longer accept donations to Wikileaks. The decision follows pressure, by US homeland security committee chair Joe Lieberman, on businesses to stop working with the whistleblower website. Meanwhile, anonymous hackers today knocked Mastercard's website offline.

Datacell chief executive Andreas Fink said his company would seek large damages from the two credit card firms.

Wikileaks Visa had already asked Datacell – based in Iceland and Switzerland – to close its account, he said. While Mastercard had not contacted the company, Fink said he had read in the newspapers about its plans to stop working with Wikileaks.

Following discussions with lawyers, Datacell had decided it “cannot honour such requests” because they were based on “untrue and unverified accusations”.

Since 10.30pm last night, Visa and Mastercard payments have been rejected on Datacell’s system, Fink said, as the credit card companies assessed whether they run legal risks by working with Wikileaks.

“Visa users have explicitly expressed their will to send their donations to Wikileaks and Visa is not fulfilling this wish,” he said.

“We strongly believe a world class company such as Visa should not get involved by politics and just simply do their business where they are good at. Transferring money.”

A comment in the Guardian newspaper today noted that while Visa and Mastercard had refused to work with Wikileaks, both accept purchases at firearms website Gun Source, as well as transferring donations to the British National Party and the Ku Klux Klan.

Fink said Visa and Mastercard “have no problem transferring money for other businesses such as gambling sites, pornography services and the like, so why a donation to a website which is holding up for human rights should be morally any worse than that is outside of my understanding.”

Visa and Mastercard have not commented on the claims. It remains unclear when Datacell will file suit.