A watchdog is threatening NHS England with legal action if it does not begin offering fertility treatments to transgender patients as standard.
Transitioning can lead to fertility loss, but many patients are not offered a chance to have eggs or sperm stored.
The Equality and Human Rights Commission said this is "outdated" discrimination as patients with other conditions routinely get that option.
NHS England says it is a matter for government ministers to decide upon.
According to the EHRC, the decision on who should be offered the extraction and storage of eggs and sperm - known as gamete extraction and storage - falls to individual Clinical Commissioning Groups (CCGs).
"But many choose not to [offer this service]" to transgender patients, the watchdog said.
The EHRC sent a pre-action letter to NHS England on Friday, which it said was the first step towards a judicial review, and asked them to make gamete extraction and storage available to anyone having treatment for gender dysphoria.
The NHS defines gender dysphoria as a condition where a person experiences discomfort or distress because there is a mismatch between their biological sex and gender identity.
Rebecca Hilsenrath, chief executive of the EHRC, said: "Our laws and our values protect those who seek treatment for gender dysphoria.
"This means that where appropriate, treatment should be made available in order to ensure that access to health services is free of discrimination.
"A choice between treatment for gender dysphoria and the chance to start a family is not a real choice."
She added: "We have asked NHS England to reflect on the true breadth of their statutory mandate and the impact on the transgender community of these outdated policies."
Decision for ministers
The NHS said the EHRC had "misplaced their fire" by targeting it rather than the government.
An NHS England spokesperson said: "Decisions on which services are commissioned by NHS England are taken by ministers based on advice from an independently chaired panel of health experts and patient representatives, using a process set out in primary legislation."
The EHRC believes it is within NHS England's legal powers to make fertility services for transgender patients a "core service", which would mean all CCGs should offer it. But the NHS says a decision by ministers is needed to allow this to happen.
NHS England have 14 days to respond to the EHRC's letter.
A spokeswoman from the watchdog added: "Their response will determine whether or not further action is necessary."
The Department of Health declined to comment.