Deutsche Bank and Barclays have failed to prevent allegations of interest rate manipulation from being included in two ongoing lawsuits.
The decision, made by the UK Court of Appeal today, means that the banks could lose millions of pounds. It could also become a test case - with other litigation over the fixing of the London interbank offered rate (Libor) being brought against banks.
Previous legal rulings have not gone as far as to say Libor was relevant to claims against banks - it has been cited only in cases where contracts have been connected specifically to the benchmark.
The banks are appealing, and in a statement today, Barclays said:
"The Court of Appeal's decision resolves two conflicting legal judgments. With or without the Libor claims, the allegations of mis-selling have no merit."
Deutsche Bank said: "We are disappointed by the court's ruling, which we will appeal. This is a long-standing case of a loan that was made and not paid back. The defendant's introduction of broad and unsupported allegations about Libor is a bid to delay payment and divert attention from its unpaid debts, which we will continue to vigorously pursue."
The news comes as banks around the world face fines from regulators for rigging interest rate benchmarks.