* Issued a notice of default to LightSquared

* LightSquared says Inmarsat not responded to issues raised

By Paul Sandle

Feb 20 (Reuters) - British satellite firm Inmarsat said 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 due under their spectrum-sharing agreement.

Inmarsat said it was scheduled to receive the payment from LightSquared, which is backed by billionaire hedge-fund manager Philip Falcone, after the completion of the first phase of the agreement.

LightSquared's scheme was dealt a major blow last week when the U.S. Federal Communications Commission said it planned to revoke permission for the company to build out its network after tests showed it would interfere with the Global Positioning System used by airlines, the military and others.

Inmarsat issued a default notice to LightSquared on Monday, giving it 60 days to make the payment before it terminated their co-operation agreement.

LightSquared, however, said final payment was not due until Inmarsat replied 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.

A person close to LightSquared said the deal with Inmarsat allowed a "cure" period, so there was no immediate default.

"We are confident we have done everything we should have done under Phase one," countered an Inmarsat spokesman.

Inmarsat also said it had started talks with the U.S. company regarding the future of the agreement.

After the FCC news, some analysts speculated that bankruptcy might be close at hand for LightSquared, especially after it had earlier warned that it would run out of money early this year.

Tim Farrar, a telecom industry consultant, said it was too soon to say whether the dispute between LightSquared and Inmarsat could trigger a default on the roughly $1.5 billion in loans that LightSquared has outstanding.

He said if there were still "pending questions" to be resolved as LightSquared alleges on the Inmarsat contract, then LightSquared's decision not to make the payment would not necessarily represent a default.

Inmarsat said its main business, which provides communications to shipping, aircraft and remote locations worldwide, was unaffected, but it could not provide any assurance that it would receive any more payments from LightSquared.

Under the terms of the deal, Inmarsat would be able to lay claim to additional spectrum if LightSquared defaulted.

Analyst Mark James at Liberum Capital said: "Note that whilst Inmarsat leased 14MHZ to LightSquared, I believe the deal break entitles Inmarsat to an extra 3MHZ of spectrum from that already owned by LightSquared."

Revenue from LightSquared has bolstered Inmarsat as its core shipping business has been hit by sailors sending email and using facebook rather than making more expensive calls as used to be the case.

Inmarsat shares closed down 1.1 percent at 477.1 pence.

• A Permanent Economic Emergency