The whistleblowing website WikiLeaks said on Monday it was suspending publishing classified US diplomatic files to focus on fundraising "to ensure our future survival".
WikiLeaks founder Julian Assange said that because of a financial "blockade" mounted by Visa, Mastercard and other companies, it was "now forced to temporarily suspend its publishing operations and aggressively fundraise".
In a press conference in London, he condemned the blockade as politically motivated and linked to the website's release of tens of thousands of classified US diplomatic cables.
Many relate to the wars in Iraq and Afghanistan while others contain frank and sometimes embarrassing assessments of world leaders made by US diplomats, prompting outrage in Washington.
Assange said WikiLeaks was taking legal action against the blockade across the world but warned that it posed an "existential threat" and if it was not lifted by the end of the year, "the organisation cannot continue its work".
The announcement appears to contradict remarks Assange made last week that WikiLeaks was managing to survive the financial blockade and was even preparing more disclosures.
Speaking to Latin American media by teleconference on October 17, he said: "We have thousands of pending disclosures for publications, we have signed contracts with more than 50 media organizations around the world."
The Australian former computer hacker is currently living under strict bail conditions in Britain while he fights extradition to Sweden, where he is wanted to answer allegations of rape.
He denies the claims against him, saying they are politically motivated.
In a statement he read out to reporters in London, Assange said that since December last year, "an arbitrary and unlawful financial blockade has been imposed by Bank of America, Visa, MasterCard, PayPal and Western Union".
"The attack has destroyed 95 percent of our revenue," he said.
"The blockade came into force within 10 days of the launch of Cablegate as part of a concerted US-based, political attack that included vitriol by senior right-wing politicians, including assassination calls against WikiLeaks staff."
He said the blockade "has cost the organisation tens of millions of dollars in lost donations at a time of unprecedented operational costs".
WikiLeaks spokesman Kristinn Hrafnsson explained that before the blockage the average donation each month exceeded 100,000 euros, but that since then contributions had plunged to between 6,000 and 7,000 euros.
Assange said: "We have commenced pre-litigation action against the blockade in Iceland, Denmark, the UK, Brussels, the United States and Australia.
"We have lodged an anti-trust complaint at the European Commission and expect a decision by mid-November as to whether the European Commission Authority will open a full investigation into the wrongdoing of Visa and MasterCard."
He added: "A handful of US finance companies cannot be allowed to decide how the whole world votes with its pocket."