The Supreme Court - the UK's highest court - has sat in secret for the first time to hear a case involving an Iranian bank and the government.
According to nine Supreme Court justices a private hearing needed to be held so that material relating to national security could be considered.
Only two lawyers joined justices at the private hearing.
One was a barrister representing the Treasury - the other a barrister who represented the interests of the bank but was not allowed to reveal what went on to bank bosses.
Journalists, the public, bank officials and lawyers representing the bank were barred.
Human rights campaign group Liberty described the move as " a sad landmark in British legal history."
Supreme Court President Lord Neuberger said justices had reached the decision with "great reluctance" but thought that a behind-closed-doors hearing was "absolutely necessary" in order to dispose of the case "justly".
Supreme Court justices were considering a case involving the Treasury and an Iranian bank accused of indirectly helping finance Iran's nuclear weapons programme.
Bank Mellat is trying to overturn an order, made by the Treasury under counter-terrorism legislation, which bars it from operating in the UK.
Bank bosses say the order is unlawful and unfair.
They have appealed to the Supreme Court after failing to persuade the High Court and the Court of Appeal to overturn the order.
When the bank tried to overturn the order in the High Court in 2010, judge Mr Justice Mitting had heard evidence behind closed doors - and delivered a secret judgment based on that evidence.
Neither Bank Mellat, its lawyers, the press nor the public was allowed into the private hearing before Mr Justice Mitting - nor allowed to read his secret judgment.
The Court of Appeal considered that secret judgment before dismissing Bank Mellat's appeal in 2011.
Treasury ministers, who have the power to impose sanctions on anyone involved in nuclear weapons development which poses a "significant risk" to the UK, had argued that the Supreme Court could and should consider Mr Justice Mitting's secret judgment before ruling on Bank Mellat's latest appeal.
Bank Mellat had argued that the Supreme Court had no statutory power to consider the secret judgment and did not need to consider the secret judgment when making a decision on whether or not to overturn the Treasury order.
Liberty had also said the Supreme Court had no statutory power to consider secret material.
Justices were asked to decide whether Parliament had given the Supreme Court the power to consider material given in secret and whether, if the Supreme Court had that power, they should consider the secret judgment given by Mr Justice Mitting.
Earlier this week the panel decided by a 6-3 majority that the Supreme Court had the jurisdiction to consider the Mr Justice Mitting's secret judgment.
And Lord Neuberger said justices had today concluded - by a majority - that they needed to examine Mr Justice Mitting's private judgment in order to make a fair decision on Bank Mellat's appeal.
Justices said they would rule on that appeal at a date to be fixed after analysing evidence at a three-day hearing.
Lawyer Sarosh Zaiwalla, who represents Bank Mellat, said later: "Britain is internationally recognised as a beacon of open justice and parties from all over the world come to the UK because of this.
"It is therefore of international concern that the United Kingdom's parliament has given the right to the court to use this closed evidence procedure."