A US judge has ordered Apple to disclose details of its patent-sharing deal with HTC to its rival, Samsung.
Apple and HTC signed a 10-year licence agreement earlier this month, but did not make the details public.
Samsung, which is also involved in various patent disputes with Apple, asked the courts to tell Apple to furnish the information.
It said it was "almost certain" the deal covered some of the patents at the centre of its dispute with Apple.
The court ordered Apple to produce a full copy of the settlement agreement "without delay", subject to an "attorneys' eyes only" designation, meaning it will not be made public.
The deal between Apple and HTC saw the two firms settle all their outstanding disputes over patents, ending a fight that began in March 2010.
According to some reports, the two companies were fighting almost 20 cases across the globe.
While that fight has ended, Apple is still involved in legal tussles with Samsung.
The two rivals have filed cases against each other in more than 10 countries, each accusing the other of violating its patents.
Earlier this year, a Californian court awarded Apple $1.05bn (£652m) in damages against Samsung, after ruling that several of its software and design technologies had been infringed.
However, the South Korean firm has appealed against the ruling and has called for a retrial.
Some analysts said that the latest decision by the court, giving Samsung access to Apple's deal with HTC, may have a big impact on Samsung's legal battle with Apple.
"It is clearly a very smart move from Samsung - because the general feeling is that a lot of its patent disputes with Apple are very likely to be similar to those between HTC and Apple," Andrew Milroy of consultancy Frost & Sullivan told the BBC.
"And if there are similarities, it gives Samsung an advantage in any future legal issues and negotiations with Apple."
The decision is the latest setback for Apple in its various legal clashes with rivals.
Last month, Apple lost its appeal against a UK ruling that Samsung had not infringed its design rights.
In a further blow, the US technology firm was asked by a UK High Court to publish a statement on its website admitting that Samsung had not infringed its designs.
Sales bans sought by Apple against Samsung's Galaxy Nexus phone and Samsung's Galaxy Tab 10.1 tablet computer in the US were also lifted in October.
Then, earlier this month, a judge in the US dismissed a case brought by Apple alleging that Google's Motorola unit was seeking excessive royalty payments for patents.
"Apple has been having a really bad time of late in its legal battles," said Mr Milroy. "They are going to have to re-examine the legal approach they take from here on."