India's government Tuesday asked opposition leader Rahul Gandhi to respond to claims that he is a British citizen as the country's fiercely contested election battle took a bizarre twist.

The home ministry wrote to the Congress party leader and scion of the famed Nehru-Gandhi dynasty after a deputy of Prime Minister Narendra Modi's ruling party accused Gandhi of lying about his nationality.

Gandhi was told to provide a ‘factual’ reply on his citizenship. India does not allow dual nationality and only Indian citizens can contest elections.

The complaint by Bharatiya Janata Party lawmaker Subramanian Swamy repeats long-standing claims that Gandhi recorded himself as British twice in annual returns filed by a now-defunct British company in which he was a director.

The company, Backops Limited, was registered in 2003 and filed for dissolution in 2009.

Swamy has been demanding since 2015 that Gandhi be stripped of Indian citizenship as well as his parliamentary seat.

Tuesday's notice is the first time the Indian government has taken up the complaint.

A Congress spokesman dismissed the allegation against Gandhi as ‘malicious’.

‘The entire world knows that he (Gandhi) is an Indian citizen by birth.... there is no truth in the allegation,’ Randeep Singh Surjewala told reporters.

Gandhi, whose mother Sonia is Italian-born and whose father was the assassinated prime minister Rajiv Gandhi, has in turn called on Swamy to substantiate his claims with documentary proof.

The allegations against Gandhi bear similarities to efforts by the so-called ‘birther’ movement, once championed by Donald Trump, that claimed then-president Barack Obama was born outside the United States and therefore ineligible.

The Indian government request came as the Gandhi-Modi battle in India's ongoing marathon general election reaches a peak.

Modi is seeking a second term as prime minister, while Gandhi is fighting to cut the government's huge majority won in 2014.

The final day of voting is on May 19 with the results due four days later.

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