The federal government is beefing up funding and extending the royal commission into trade unions to further probe evidence of widespread criminal conduct.

Labor and the peak union body call the decision “transparently political” with the reporting date extended to December 2015 despite no official request from Royal Commissioner Dyson Heydon.

But Attorney-General George Brandis brushed off suggestions it will enable the government to release the report in an election year.

“We’re responding to what the royal commissioner has said to us,” he told reporters in Canberra on Tuesday.

He released a letter from Commissioner Heydon that indicated there was a lot of unfinished business for the royal commission to look at before its current December 31 deadline.

Commissioner Heydon’s letter says the inquiry revealed evidence of criminal conduct, including widespread instances of physical and verbal violence, cartel conduct, secondary boycotts, contempt of court and other orders, and the encouragement of others to commit these contempts.

ACTU assistant secretary Tim Lyons pointed to the final paragraph of the letter, which reads: “This letter is neither an application to widen the terms of reference nor an application to extend the reporting date.”

“It’s entirely unprecedented as far as we know for a government to extend a royal commission even though the commissioner didn’t explicitly ask for it,” Mr Lyons said.

The letter says the commission would meet its deadline and fulfil its terms of reference.

Shadow attorney-general Mark Dreyfus says the extension doesn’t change what this Royal Commission is – “a desperate and shameless use of executive power for the government’s own political advantage”.

Senator Brandis denied the Abbott government was going down the American path of prosecuting previous governments for political purposes.

Last month former Liberal prime minister John Howard spoke of his unease about the coalition government holding commissions into trade unions and the Home Insulation Program.

“I agree with what Mr Howard said,” Senator Brandis said.

“But the point I’d make is this is not about politics. This is about the criminal law and the compliance by important and powerful institutions in our society.”

The commission will have until December 31, 2015 to report and $8 million in additional funding on top of $53 million already allocated to investigate evidence of criminal conduct.

The peak builders’ body welcomed the extension.

“To more thoroughly examine these matters and recommend reforms necessary to enforce the rule of law in the construction industry,” Master Builders Australia CEO Wilhelm Harnisch said.

The 12-month extension means the Heydon royal commission will run for more than 20 months.

By David Barbeler and Peter Trute