That decision blocked her entitlement to the pension when she reached 60.
Supreme Court judges said they were divided on the issue and would look to the EU Court of Justice for guidance.
MB lost her case at the Court of Appeal in 2014 when judges upheld a decision by the Department for Work and Pensions (DWP) to refuse her a female pension on the basis that she remained legally a man.
‘Sight of God’
Under the 2004 Gender Recognition Act, trans people acquired the right to formally change their gender by obtaining a full “gender recognition certificate”.
But a certificate could not be issued to a married person who did not have their marriage annulled on the basis of their gender change.
MB, who married in 1974, began to live as a woman in 1991 and underwent gender reassignment surgery in 1995, but did not apply for a gender recognition certificate.
She says she preferred to stay married to her wife and the mother of their two children “in the sight of God”.
When she reached her 60th birthday in May 2008, MB applied for a state pension, but was refused on the basis that legally she was still a man, and should therefore wait for the male pension at 65.
When the Court of Appeal rejected her attempt to challenge that, Lord Justice Maurice Kay described MB as the victim of “a real misfortune” and said changes in the law had occurred “too late for her to benefit from them”.
‘Matter of principle’
MB had asked the Supreme Court justices in London to overturn the Court of Appeal’s decision. Her lawyers argue that the DWP’s reliance on domestic UK pensions legislation is in contravention of EU laws.
The Supreme Court’s deputy president, Lady Hale, said the court was “divided” on the correct answer.
“Since there is no CJEU (Court of Justice of the European Union) authority directly in point, it refers the question for their guidance,” she added.
Christopher Stothers, a partner at law firm Arnold & Porter, who represents MB, said: “This issue is a matter of principle as well as having financial consequences for pensioners.
“Where an individual is physically, socially and psychologically a woman, as recognised by the state in their passport and driving licence, and indeed surgically, why should they be required – before the state will recognise their gender for pension purposes – to get divorced or have their marriage annulled, particularly where they and their spouse do not wish to do so and indeed have religious objections to doing so?
“Although we are pleased with the result, the slowness in getting the issue resolved is highly frustrating for the pensioners involved.”