The British satellite company Inmarsat said yesterday its partner LightSquared had missed a $56.3m (£35.5m) payment.
Inmarsat was scheduled to receive the payment from LightSquared, which is struggling to build a US mobile broadband service with Inmarsat's spectrum, after the completion of the first phase of their deal.
However, LightSquared said thepayment was not due until Inmarsat replies to several issues it had raised.
Inmarsat said it had issued a default notice to the US group, backed by the billionaire hedge-fund manager Philip Falcone, giving it 60 days to pay before it terminates the deal.
The satellite company said its core business, which provides communications to shipping, aircraft and remote locations worldwide, was unaffected, but could not provide any assurance that it would receive any more payments from LightSquared.
Last week, the US Federal Communications Commission said it planned to revoke permission for LightSquared to build its network after tests showed it would interfere with GPS used by airlines and others, fuelling speculation that bankruptcy could be lose for the group.