More than 60 Australian federal politicians have explicitly called on the US to drop the prosecution of Julian Assange, warning of “a sharp and sustained outcry in Australia” if the WikiLeaks founder is extradited.

With a small cross-party delegation due to fly to Washington next week, the Guardian can reveal that the lobbying trip has won the open support of 63 members of Australia’s House of Representatives and Senate.

In a letter, the 63 MPs and senators said they stood in support of the trip to the US and were “resolutely of the view that the prosecution and incarceration of the Australian citizen Julian Assange must end”.

The Australian politicians noted “with gratitude the considerable support in the United States for an end to the legal pursuit of Mr Assange from members of Congress, human rights advocates, academics, and civil society, and from within the US media in defence of free speech and independent journalism”.

“On that basis we ask Congresspeople, members of the press, and other relevant civil society stakeholders in the United States to speak up now in supporting an end to the prosecution and detention of Julian Assange,” they wrote.

Alternatively, they said, “a decision to simply abandon the extradition proceedings would have the sensible, just, and compassionate effect of allowing Mr Assange to go free from a prolonged and harsh period of high-security detention”.

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