The ABC has confirmed former prime minister Malcolm Turnbull will resign from Parliament later this week.

Mr Turnbull lost support in the Liberal partyroom last week and stepped aside from the top job.

For at least two sitting weeks, the Federal Government will hold 75 of 149 seats in the House of Representatives.

Mr Turnbull will resign on Friday, triggering a by-election in his Sydney electorate of Wentworth.

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Christine Forster — the sister of former prime minister Tony Abbott — has confirmed she wants to be the Liberal Party's candidate for the seat.

Ms Forster has been a Liberal councillor for the City of Sydney for the past six years and was a leading figure in last year's successful same-sex marriage campaign.

The by-election date will be set by the Speaker, Tony Smith, after Mr Turnbull formally resigns.

Mr Turnbull now holds the seat on a margin of 17.7 per cent.

But the margin was just 5.5 per cent when he claimed the inner-Sydney electorate in 2004, giving hope to Labor, the Greens and independents.

Earlier on Monday, Mr Turnbull's son-in-law James Brown ruled out running for the seat.

"Serving in the Australian parliament would be an enormous honour, but whenever Malcolm Turnbull resigns I will not be standing for preselection in Wentworth," the NSW RSL president said.

"Thanks to those who have kindly suggested I should," he said.

Mr Turnbull has previously said he would swiftly exit Parliament if he lost the leadership.

"I'll be leaving the Parliament … not before too long," Mr Turnbull said last Friday in his final media conference as prime minister.

"I've been very clear about that. It's not a secret."

In a letter to his eastern Sydney electorate, Mr Turnbull thanked the community for allowing him to represent them for 14 years, including three years as prime minister.