Deelavathi Bondada, the daughter of an Indian immigrant who settled in Britain and became a citizen of the UK, had requested a British passport from the UK home office to escape an abusive marriage in India.
However, UK immigration authorities had denied her request based on some doubts over her paternity.
A British judge has now ruled those doubts were "untenable" and granted the 45-year-old British citizenship on the basis of her descent.
Justice Walker was asked to rule on the dispute between the Home Office and Bondada, who was born in Indiain 1969, two years after her father Chandraiah became a British citizen.
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The ruling earlier this week quashed the Home Office's October 2013 decision refusing her one.
Walker said government officials who objected to Deelavathi's claim had ignored "compelling DNA evidence".
He said the unsupported speculation that her mother, Ganikamma, who was now 86, had had a secret lover who fathered Deelavathi was "so far-fetched as to be absurd".
"It is not a real possibility, let alone a possibility of such substance as to enable the court to make a finding that Deelavathi has not shown on the balance of probability that Chandraiah was her father," the judge said in a written ruling.
"Nevertheless the stance taken on behalf of the home secretary when rejecting (Deelavathi's) claim has, without a shred of evidence to support it, continued to make the same effective accusation. The conduct of the UK government in this regard has been grotesque."
The judge said lawyers representing the Home Office were not to blame but had been asked to "defend an impossible position".
He had analysed "crucial issues" about her date of birth and her parents' marital status before concluding that a decision to refuse her a passport should be quashed.