Prime Minister Tony Abbott has found an unlikely defender against online attacks claiming he is a dual citizen and ineligible to hold office.

Labor MP Tim Watts has hit out at the "Australian birthers" and labelled their allegations a "conspiracy theory" aimed at undermining confidence in Australia's political system.

Tony Abbott's office deny the Prime Minister holds dual citizenship. Credit:Alex Ellinghausen

The so-called Australian "birthers", a nod to similar US right-wing attacks against President Barack Obama, have been claiming in blogs and on social media for months that Mr Abbott, who was born in London but emigrated to Australia as a child, is a dual citizen.

Section 44 of the constitution forbids dual citizens from becoming MPs and states that any person who is a "subject or a citizen . . . of a foreign power" shall be "incapable of being chosen" as a member of either house.