Labor has called on the Turnbull government to refer Liberal MP John Alexander to the High Court amid mounting evidence he is a dual citizen.

Citizenship experts agree Mr Alexander is likely to be a UK citizen by descent after Fairfax Media revealed his father was born in England before moving to Australia as a young child. Mr Alexander has never renounced UK citizenship and cannot say for sure whether his father did so before he was born.

Experts have also challenged the backbencher's claim that his father had 40 years in which to renounce, confirming the window was actually just two years.

Prime Minister Malcolm Turnbull on Tuesday said Mr Alexander has told the party he "believes" he is not a dual citizen.