Former prime minister Julia Gillard is set to appear as a witness before the royal commission into union corruption on Wednesday.

She is the only witness listed for the day, suggesting she may face an intense session of questions.

The former Labor leader is expected to be grilled over a slush fund operated by her former boyfriend Bruce Wilson when he was an official with the Australian Workers’ Union in the early 1990s.

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Ms Gillard was a lawyer with the firm Slater and Gordon at the time and did work advising the union, including advising on the setting up of the fund.

Witnesses at the commission have testified Mr Wilson gave Ms Gillard money from the slush fund to pay for renovations to her house. Ms Gillard has repeatedly denied the claims.

Ms Gillard’s former boss at Slater and Gordon, Judge Bernard Murphy, will be called as a witness on Tuesday.