The UK's highest court has been asked to rule on a transgender person's claim to be entitled to receive the female state pension at the age of 60.
Five Supreme Court justices will give their decision on a date to be fixed after a day of legal submissions on Tuesday.
A ruling will be made by the court's deputy president Lady Hale, Lord Wilson, Lord Sumption, Lord Toulson and Lord Hodge.
MB, who cannot be identified, transitioned from male to female but decided as a Christian to stay married "in the sight of God" to her wife and mother of their two children - a decision that blocked MB's entitlement to the pension.
The Court of Appeal rejected the claim in July 2014. Appeal judge 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 judge added it was "in the nature of such changes" in social attitudes that they would "come too late for some".
Lord Justice Kay, sitting with Lord Justice Aikens and Lord Justice Underhill, upheld a decision of the Department for Work and Pensions refusing a female pension.
MB, who is now 68, married while still a man in 1974. She began to live as a woman in 1991 and underwent gender reassignment surgery four years later.
In April 2005 trans people 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.
The appeal judge said MB "does not wish to have her marriage annulled".
He said: "Also, as a Christian she says that she and her wife feel married in the sight of God. Accordingly she has not applied for a gender recognition certificate, and so far as the law is concerned she remains a man."
When she reached her 60th birthday in May 2008, MB 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.
The appeal judges unanimously declared that the refusal did not contravene the principle of equal treatment and was not discriminatory.
MB has asked the Supreme Court justices to overturn the Court of Appeal's decision.