* Inmarsat says payment of $56.25 mln not received
* Issued a notice of default to LightSquared
* LightSquares says Inmarsat not responding to issues it raised
Feb 20 Satellite firm Inmarsat said its partner LightSquared, a venture struggling to build a U.S. mobile broadband service with Inmarsat's spectrum, had failed to pay a $56.25 million instalment to the British company.
Inmarsat was scheduled to receive the payment from LightSquared after the completion of the first phase of their agreement.
However, LightSquared said final payment was not yet due until Inmarsat replies to several issues that it had raised.
"LightSquared has raised several matters that require resolution before the first phase comes to a close," it said in a statement.
The terms of the agreement between the two allow for additional time to resolve pending questions, the broadband service provider said.
Earlier, Inmarsat said it had issued a default notice to LightSquared on Monday, giving it 60 days to make the payment before it terminated their co-operation agreement.
Inmarsat also said it had started talks with the U.S. company, backed by billionaire hedge-fund manager Philip Falcone, regarding the future of the agreement.
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 U.S. Federal Communications Commission said it planned to revoke permission for LightSquared to build out its network after tests showed it would interfere with the Global Positioning System used by airlines, the military and others.
After the FCC news, some analysts speculated that bankruptcy may be close at hand for LightSquared, especially after it had earlier warned that it would run out of money early this year.
Shares of Inmarsat fell 2 percent to 473.1 pence on Monday before paring some losses to trade at 478.5 at 1402 GMT.