Julian Assange, the leader of Wikileaks, will have been detained in the UK for 500 days on April 20. Credit: Fabrice Coffrini, file photo (AFP)

A viral Tweet which alleges Australia's Prime Minister and Attorney General ordered Mastercard to block donations to Wikileaks has been rubbished by Mastercard.

The Tweet, which also gave the name and phone number of a Mastercard executive, went viral after being retweeted by Wikileaks supporters and Wikileaks itself. However, the same executive subsequently told ABC Technology, "That is absolutely not what I said" and that it was "Absolutely our decision given the serious nature of the allegations".

The story broke as Wikileaks and its supporters plan to mark Julian Assange's detention in the UK for 500 days without charge, on April 20.

[Update] It transpires that the Tweet was based upon and email which describes a conversation between a Wikileaks supporter and the Mastercard executive. The email states, "He said that they have every right to stop dealing with whoever they want to. He also said that they acted on the instructions of our PM and our Attorney general, but he added that they are not blocking donations for Julian Assange's legal fees. It would be good if others would ring him too."

However, a further statement from the Mastercard executive reads: "The decision to suspend acceptance to Wikileaks was MasterCard's alone. Given the serious nature of the allegations and some of the public comments by senior elected officials from around the world (including the Australian Prime Minister and the then Attorney-General who indicated that there were questions about the legalities of Wikileaks activity) we believed it was prudent to suspend acceptance. The allegation that we were instructed by the PM and AG is incorrect."

[Update 2] Readers have been asking whether there was any discussion at all between Mastercard and the government before the decision was made. Mastercard has subsequently told ABC Technology that "there was no contact at all between MasterCard and the Government before our decision."