Thirty men have been stopped from donating blood at clinics in Northern Ireland because they have had sexual contact with another man, a BBC investigation has found.
Elsewhere in the UK, there is a one-year deferral period for men who have had sex with men (MSM) to donate.
Northern Ireland has an outright ban.
But a judge ruled that former health minister Edwin Poots did not have the power to retain that ban. His ruling will be appealed in court later.
In the Northern Ireland Appeal Court on Monday, the current on-off health minister, the Democratic Unionist Party's (DUP) Simon Hamilton, is appealing the judgement, alongside Health Secretary Jeremy Hunt.
They shall be asking who is in charge of blood policy and whether or not this is a devolved issue. The appeal is expected to last four days.
Following a Freedom of Information request, BBC News NI has seen emails sent between Northern Ireland's Department of Health and the Northern Ireland Blood Transfusion Service (NIBTS), which is responsible for the collection, testing and distribution of blood.
The NIBTS also said it had stopped 30 men from donating blood at their clinics since 2011 after they informed staff that they had sexual contact with other men.
The BBC asked the NIBTS how confident it would be that its blood is screened correctly and that it would be safe for MSM to donate after the one-year deferral period.
It said: "All blood donations are subjected to the testing regimes required by the Blood Safety and Quality Regulations 2005.
"As such, NIBTS is confident that all blood samples are screened correctly."
The BBC has also seen instructions sent to the NIBTS from Dr Elizabeth Mitchell, the deputy chief medical officer, instructing the organisation how to respond if approached by the media about the ban remaining in place.
Dr Kieran Morris, the former chief executive of the NIBTS, replied showing some concern about the process and how he would answer questions from his own staff.
"As chief executive officer and accountable officer for the NIBTS special agency service, I require from the Department of Health a written direction, giving me a clear line as to how we manage and control the situation," he said.
"There is no doubt in my mind that referring all matters to the Department of Health press office will not be sustainable for more than a few days."
A BBC investigation earlier this year found the Department of Health does not have any medical evidence of its own to support a permanent ban on gay men donating blood.
The ban was put in place across the UK during the Aids crisis of the 1980s, but was lifted in England, Scotland and Wales in November 2011.
New rules were introduced that allowed blood donations from men whose last sexual contact with another man was more than a year earlier.
But Northern Ireland did not follow suit.
A gay man, granted anonymity due to his perceived vulnerability, launched a judicial review challenge over then health minister Edwin Poots's decision not to adopt the same policy on this side of the Irish Sea.
Mr Poots said he had kept the ban on the basis of ensuring public safety.
In April, the European Court of Justice (ECJ) ruled that a lifetime ban may be justified in member states if no effective detection techniques exist within the country.
The ECJ said countries must establish if such donors were at high risk of acquiring infectious diseases like HIV.
Mr Hamilton said he would study the ruling.
A number of issues will be looked at in the Court of Appeal, including whether blood policy should be a devolved matter.
The appeal is expected to last for four days.
The NIBTS did not respond to the BBC to give an additional comment.
A Department of Health spokesman said: "It would not be appropriate at this stage to comment on matters that are before the courts."