LONDON - WikiLeaks founder Julian Assange issued a plea from jail for his supporters to keep fighting as the British Government braced for a likely attack on government websites.

Extra security measures were added to a host of British government web services, in particular those used to claim benefits or provide tax information, after Sir Peter Ricketts, the national security adviser, warned permanent secretaries across all departments in Whitehall that "hacktivists" who last week targeted the sites of companies such as MasterCard and PayPal could switch their focus to Britain.

Downing St officials confirmed they believed today's court appearance by Assange - in custody after his arrest on sex allegations at the request of the Swedish authorities - could be used by hackers as an excuse to switch their focus to key cyber infrastructure such as the website of HM Revenue and Customs.

Members of the online collective Anonymous have already signalled their willingness to attack British targets if Assange - who denies the claims and whose lawyers will today apply for bail - is extradited to Sweden. Almost 600,000 people have signed an online petition in support of WikiLeaks.

The Sunshine Coast Daily reported that Assange had said he was determined to fight for the future of WikiLeaks.

"My convictions are unfaltering. I remain true to the ideals I have always expressed," he said in a statement from Wandsworth Prison in London, conveyed by his mother Christine.

"These circumstances shall not shake them. If anything, this process has increased my determination that they are true and correct. We now know that Visa, Mastercard and Paypal are instruments of US foreign policy. It's not something we knew before. I am calling on the world to protect my work and my people from these illegal and immoral acts."

Christine Assange was not able to see her son face-to-face yesterday but a 10-minute phone call reunited the duo. It was the first time the Noosa woman had spoken to her son since he was arrested.

Assange told his mother there were CCTV cameras monitoring his cell because of fears he could be assassinated for his role in releasing 250,000 confidential US government documents.

The ability of amorphous groups such as Anonymous to disrupt and paralyse websites was displayed again yesterday when hackers obtained the passwords of 1.3 million users of the gossip website Gawker and posted them online. The motivation for the attack, claimed by a group calling itself Gnosis, was not clear, but Gawker has previously published blogs criticising Assange and 4chan, the messaging board that spawned Anonymous. In the wake of the attack, Gawker's Twitter accounts were hijacked to publish messages supporting WikiLeaks.

Amazon, the world's biggest online retailer, insisted yesterday that the disappearance of its European websites for about 30 minutes on Monday was due to a "hardware failure". The company is one of those which had been threatened as part of Operation Payback, the attempt by Anonymous to mount attacks against companies which withdrew services from WikiLeaks.

The anger of Assange's supporters is likely to be increased by a claim from his British lawyer yesterday that a grand jury has been secretly empanelled in Virginia to consider charges against the Australian over the leaked diplomatic telegrams.

IT experts have warned that Whitehall is particularly vulnerable to cyber attacks because many computers still run on an outdated version of Internet Explorer known to be at particular risk to hackers. The Government has ruled out an upgrade on the grounds of cost.

In newly released cables published by WikiLeaks, it was revealed that British police helped to "develop" evidence against Madeleine McCann's parents as they were investigated by Portuguese authorities looking into their daughter's disappearance.

Britain's ambassador to Portugal, Alexander Wykeham Ellis, reportedly made the claim to his American counterpart on September 21, 2007 - two weeks after Portuguese police named Gerry and Kate McCann as "arguidos", or formal suspects.

In a cable to Washington, US Ambassador Al Hoffman wrote: "Without delving into the details of the case, Ellis admitted that the British police had developed the current evidence against the McCann parents, and he stressed that authorities from both countries were working co-operatively."

The cable does not specify what evidence British police are alleged to have gathered, or whether UK investigators were involved in the decision to formally name the McCanns as suspects. They remained under official suspicion until July 2008 when Portuguese police shelved the investigation into Madeleine's disappearance.

- Independent, AFP, PA