Pressure is mounting on the head of the royal commission into trade unions, Dyson Heydon, who has conceded he “overlooked” the connection between the Liberal Party and a speech he had agreed to give.

Prime Minister Tony Abbott and Justice Heydon – the man his government appointed to lead the royal commission – will anxiously wait to see if the Australian Council of Trade Unions (ACTU) will disqualify the former High Court judge.

They gave the ACTU until Thursday to make its decision whether to seek Justice Heydon’s removal from the controversial inquiry into union corruption over perceptions of bias.

• ACTU to decide on Heydon disqualification
• Calls for Royal Commissioner to go amid fundraising row
• ALP wants Heydon’s Liberal invite

After the ACTU interrupted the Royal Commission into Trade Union Governance and Corruption’s proceedings to discuss the matter on Monday, Justice Heydon said that to the best of his knowledge he first informally discussed giving the speech “several years ago”.

“The email stated that it was organised by a body which I was told was one of the lawyer branches of the Liberal Party NSW division which had a focus on professional engagement,” Justice Heydon said.

However, he did not understand the event to be a party fundraiser.

Labor and the government are locked in an arm wrestle over Dyson Heydon and the union royal commission.

Justice Heydon said he could only deliver the address if the royal commission had finished, which at that stage was due to be December, 2014, but was extended by one year.

He said he had “overlooked” the Liberal connection when he received other emails about the event earlier this year.

It was not until last week – when he read the latest email from organisers – that his office wrote back: “If there is any possibility that the event could be described as a Liberal Party event he will be unable to give the address, at least whilst he is in the position of Royal Commissioner.”

Justice Heydon decided not to speak at the event shortly after sending the email.

The ACTU on Monday sought the release of the emails to help it consider whether to seek Justice Heydon’s removal on the grounds of bias.

Counsel assisting the commission Jeremy Stoljar said the application “smacks of grandstanding”, and the commissioner initially gave the ACTU only an hour to consider the emails.

But the ACTU’s counsel Robert Newlinds said it was a serious matter and required a careful analysis of the emails.

In parliament, Mr Abbott described Justice Heydon as professional and impartial.

“It’s never been disputed that this was a Liberal Party event,” Mr Abbott said.

The Opposition believe Justice Heydon agreeing to give the Sir Garfield Barwick oration is the “smoking gun” it needed to prove the commission was politically motivated.

Labor spokesman Tony Burke said during question time on Monday that the government must “now scrap” the royal commission.

ACTU secretary Dave Oliver said on Monday there was at least an appearance of bias because Justice Heydon knew the speech was a Liberal event, and he had overlooked the fact it was a fundraiser.

Rhodes connection

It was also revealed on Monday that Mr Abbott had a personal connection with Justice Heydon going back decades.

The connection dates back to Mr Abbott’s selection as a Rhodes scholar in 1980, as a 23-year-old student at Sydney University, the ABC reported.

Documents at the NSW state archives confirmed a ‘Professor JD Heydon’ was a member to the committee that selected Mr Abbott for the prestigious scholarship, that saw him attend Oxford University.

A document shows Dyson Heydon’s position on the Rhodes Scholarship selection committee in 1980. Photo: ABC

During a press conference on Monday morning, Mr Abbott was questioned about the Rhodes link, but said he could not recall whether Justice Heydon was on the committee.

“That’s a long time ago and the idea that I gather is being peddled that somehow he and I cooked up a conspiracy 34 years ago against the Labor Party is absurd,” Mr Abbott said.

– with AAP, ABC