LightSquared has defaulted on a US$56.25 million payment due under a 2007 wireless spectrum cooperation agreement with Inmarsat, the U.K. satellite communications operator said Monday, adding that it could terminate the agreement if LightSquared doesn't make payment within 60 days.

The future of LightSquared's project to offer LTE (Long Term Evolution) broadband wireless services across the U.S. using a combination of satellite and terrestrial signals was thrown into doubt last week, when the U.S. Federal Communications Commission said it was minded to withdraw its approval for the project. LightSquared is considering a legal challenge to the FCC's decision.

The FCC had initially made its approval conditional on LightSquared working with the GPS industry to reduce concerns over interference with GPS navigation devices, which operate on frequencies adjacent to those LightSquared intends to use. LightSquared's spectrum cooperation agreement with Inmarsat is key to the proposals it made to solve the GPS interference issue: Of the frequencies LightSquared could use for its network, Inmarsat controls the block furthest from that used by GPS receivers, and so the least likely to cause interference problems.

LightSquared has already invested $4 billion in building its network, and will need to invest another $14 billion in private financing to complete its plans, it says.

In early January the company said it had enough financing to wait several more quarters for regulatory approval to begin operating its network, but Monday's default on the payment to Inmarsat suggests LightSquared's finances may be tighter than the company has previously let on.

As of Nov. 2, LightSquared had paid Inmarsat $420.6 million under their spectrum cooperation agreement, of which $268.1 million was paid last year, according to Inmarsat's financial statements. Following LightSquared's failure to pay Monday's $56.25 million installment, Inmarsat said it had given the company notice that it has 60 days to pay, after which Inmarsat can terminate certain of LightSquared's rights to spectrum set out in their cooperation agreement.

Inmarsat warned its investors that although it is negotiating with LightSquared, it cannot be sure that it will receive any further payments from the company.

Peter Sayer covers open source software, European intellectual property legislation and general technology breaking news for IDG News Service. Send comments and news tips to Peter at peter_sayer@idg.com.