A group of federal MPs from across the political spectrum will travel to Washington this month to stop the United States’ ongoing pursuit of Wikileaks founder Julian Assange.

The delegation includes Nationals MP Barnaby Joyce, Labor’s Tony Zapia, Independent MP Monique Ryan, Liberal Senator Alex Antic, and Greens senators Peter Whish-Wilson and David Shoebridge.

They plan to lobby their counterparts to abandon attempts to extradite Assange, 52, to the US in relation to historic allegations of espionage during their September 20-21 visit.

Their trip is aimed at drawing attention to Assange’s circumstances in the month leading up to Prime Minister Anthony Albanese’s visit to the White House at the end of October.

Assange, an Australian citizen, published a trove of classified documents more than a decade ago.

He has been in prison in the United Kingdom for more than four years and is fighting extradition to the US.

“Australians are united in their view that this matter must come to an end now,” Australian human rights lawyer and Assange campaign advisor Greg Barns SC said in a statement on Tuesday.

“Julian should be immediately reunited with his wife and children. That can only happen if the US Department of Justice drops its extradition bid at once.”

Assange’s brother Gabriel Shipton said the US administration was being “vengeful”.

“Even Australians who didn’t support Julian’s actions believe he has suffered enough and should be set free immediately,” he said.

The news of the delegation comes after nine former attorneys-general – including those at a federal, state and territory level – wrote to Prime Minister Anthony Albanese last month, labelling Assange’s treatment as “deeply troubling”.

The signatories of the joint letter include former federal attorney-general Michael Duffy, former ACT attorney-general Bernard Collaery, former Victorian attorney-general Rob Hulls and former NSW attorney-general Bob Debus.

Since winning office in 2022, the Albanese government has been advocating for the US pursuit of Assange to end.

Assange faces 17 charges of espionage and one charge of computer misuse over WikiLeaks’ publication of hundreds of thousands of classified diplomatic and military documents in 2010.

American prosecutors allege he helped US Army intelligence analyst Chelsea Manning steal classified diplomatic cables and military files that WikiLeaks later published, putting lives at risk.

The delegation will meet with members of Congress and Senate, the US State Department and the Department of Justice.

They will also meet with think tanks and organisations including the American Civil Liberties Union, the Foundation for Individual Rights and Expression, the Committee to Protect Journalists and Reporters Without Borders.