The case of a transgender person in England who was refused the female state pension at the age of 60 after she chose to stay married is to be looked at by judges in Europe.
The move follows a hearing at the UK's highest court last month in the case of MB, who transitioned from male to female but decided as a Christian to stay married "in the sight of God" to her wife and the mother of their two children - a decision that blocked her entitlement to the pension.
Five Supreme Court justices announced on Wednesday that they were referring the legal question raised in the proceedings to the Court of Justice of the European Union (CJEU) "for their guidance".
MB, who is now 68 and cannot be identified, lost her case at the Court of Appeal in 2014 when judges upheld a decision of the UK's Department for Work and Pensions (DWP) refusing a female pension.
In April 2005 trans people in England acquired the right to apply for a full "gender recognition certificate" under the 2004 Gender Recognition Act.
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, did not apply for a gender recognition certificate.
When she reached her 60th birthday in May 2008, MB - who began to live as a woman in 1991 and underwent gender reassignment surgery in 1995 - applied for a state pension but was refused on the basis that she was a man and would have to wait for the male pension at 65.
When the Court of Appeal rejected the claim, 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".
The appeal judges unanimously declared that the refusal did not contravene the principle of equal treatment and was not discriminatory.
MB had asked the Supreme Court justices at a hearing 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, alond with Lord Wilson, Lord Sumption, Lord Toulson and Lord Hodge, said the court was "divided" on the correct answer to the question raised in MB's challenge, and that "since there is no CJEU authority directly in point, it refers the question for their guidance".
The question posed is whether European law "precludes the imposition in national law of a requirement that, in addition to satisfying the physical, social and psychological criteria for recognising a change of gender, a person who has changed gender must also be unmarried in order to qualify for a state retirement pension".
Christopher Stothers, a partner at law firm Arnold & Porter, who represents MB, said: "We are pleased with today's outcome. This case has been running since 2008. The Government refused to pay MB her pension when she turned 60, and the three lower tribunals and courts who looked at this agreed.
"However, the Supreme Court has now recognised that this is a difficult question on which the judges are divided, and so has referred a question to the European Court of Justice in Luxembourg where the question will be answered before returning to the Supreme Court in 18-24 months."
He added: "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.
"There are various related cases which have all been delayed pending the outcome of the Supreme Court judgment."
He added: "Although the numbers are small - those affected potentially in the hundreds, with cases in the tens - this is an important test of the UK's commitment to diversity and inclusivity."
Lord Sumption, giving the judgment of the court, said: "At the time which is relevant to this appeal, the acquired gender of a transsexual person was not recognised for the purpose of determining the qualifying age for a state pension, if that person was and remained party to a subsisting marriage."
When the Gender Recognition Act was passed "a valid marriage could subsist in law only between a man and a woman".
Under the provisions of the Marriage (Same Sex Couples) Act 2013, which came into full force in December 2014, a full gender recognition certificate can be obtained without a marriage having to be annulled, provided the applicant's spouse consented.
But those provisions are not retrospective and do not give MB any right to a pension from the age of 60.
Lord Sumption said: "MB has not applied for a gender recognition certificate since the coming into force of the Gender Recognition Act. This is because she and her wife continued and still continue to live together and wish to remain married.
"For religious reasons they are unwilling to see their marriage annulled, even if it can be replaced by a civil partnership."