Liberal backbencher John Alexander is making urgent enquiries to determine whether he is a British citizen by descent.

The Sydney-based MP's father was born in the United Kingdom, but Mr Alexander believes he renounced his citizenship after he migrated to Australia.

"He became an Australian citizen as soon as he could," Mr Alexander said in a statement.

"I understand he renounced his British citizenship before I was born because he was a proud Australian."

Mr Alexander has contacted British authorities for confirmation.

The ABC understand Prime Minister Malcolm Turnbull, who announced a new citizenship disclosure process early on Monday, was told about Mr Alexander's situation on Monday afternoon.

Mr Turnbull said he received advice from the federal director of the Liberal Party, Andrew Hirst, that all Liberal MPs believed they were compliant with the constitution.

Mr Alexander's father, Gilbert Alexander, was born in 1907 and migrated to Australia in 1911.

The Member for Bennelong was born in 1951, two years after the creation of Australian citizenship in 1949.

If Gilbert Alexander did not renounce his British citizenship within that two-year window, it could have passed to his son.

While announcing the new disclosure measure, Mr Turnbull said some politicians could still acknowledge they are in breach of the constitution and have to resign from Parliament.

Liberal backbencher Craig Kelly said he was not aware of Mr Alexander's situation, but said it could prompt a by-election and possibly an early federal election.

"If there are several people that are caught by this it could lead to an early election," he said.

"But we can't worry about that, our job as Members of Parliament is to put the constitution first."