LONDON -- Britain's national security adviser has warned that government websites are at risk of cyber attack from pro-WikiLeaks hackers, prime minister David Cameron's office said Monday.
Ministries have been told they could be targeted by online "hacktivists," following attacks on companies including MasterCard Inc., Visa Inc. and PayPal Inc., which cut ties to the WikiLeaks site.
Cameron's office said security adviser Peter Ricketts has raised his concerns before an extradition hearing scheduled for Tuesday, when WikiLeaks founder Julian Assange will appear at a London court.
Assange is wanted by authorities in Sweden for questioning over two sex crime allegations. Lawyers have said he denies the allegations and is fighting extradition.
Last week, a judge denied him bail and he is being held in prison pending Tuesday's court hearing.
Outside Sweden's embassy in London, about 15 people wearing masks bearing Assange's face demonstrated against the demand for his extradition. Protests are planned outside the court Tuesday.
Cameron's spokesman Steve Field said the government's priority is websites dealing with information that belongs to members of the public. He said the government has particular concerns about websites used to file tax returns or to claim benefits, which store sensitive personal information.
A loose-knit group of activists who use the name "Anonymous" has launched attacks on companies that have severed links with WikiLeaks since it began publishing its trove of about 250,000 secret U.S. diplomatic cables last month.
The disclosures, which continued late Sunday, have offended U.S. allies and angered its rivals. Officials in Washington claim other countries have already curtailed their dealings with the U.S. government because of WikiLeaks' actions.
The decision by companies including Visa and MasterCard to halt cooperation with WikiLeaks has hampered the organization's ability to accept donations. Earlier this month, Amazon.com Inc. ousted WikiLeaks from its servers, citing a violation of its terms of service.
Though Amazon sites were offline for about half an hour in Britain, France, Germany and other countries late Sunday, the company denied that was because of hacker attacks. Amazon said Monday the problem "was due to hardware failure" at its European data center.
Though their websites were briefly inaccessible last week, both MasterCard and Visa said cardholders' accounts were not at risk. PayPal also confirmed it had faced "a dedicated denial-of-service attack."
The Swiss postal system's financial arm, Postfinance, which shut down Assange's bank account, also said its website struggled last week to contend with an unexpected barrage of traffic.
Assange's legal team said he would be represented at a court hearing Tuesday by lawyer Geoffrey Robertson, a former appeals judge at the U.N. Special Court for Sierra Leone who has specialized in freedom of speech cases. Former clients include author Salman Rushdie.