By Francesco Guarascio

BRUSSELS (Reuters) - The euro zone's rescue fund, Greece's largest creditor, on Friday declared Athens in default on its euro zone loans after missing a payment to the IMF, and reserved the right to call in 130.9 billion euros in debt ahead of schedule.

The decision came two days before Greeks were set to vote in a referendum on whether to accept bailout terms rejected by their leftist government, with a 'No' vote likely to put Greece on a path to leaving the euro zone.

The European Financial Stability Facility (EFSF) said Tuesday's failure to pay the International Monetary Fund "results in an event of default by Greece, according to EFSF financial agreements with Greece".

The board of the EFSF decided to reserve its right to act at a later stage on its own outstanding loans to Greece, an EFSF statement said.

EFSF chief executive Klaus Regling had had two other options: to waive the debt, or to demand immediate repayment, which would have bankrupted Greece and forced euro zone governments to take large losses.

"This event of default is cause for deep concern," Regling said in the statement. "It breaks the commitment made by Greece to honor its financial obligations to all its creditors and it opens the door to severe consequences for the Greek economy and the Greek people."

Experts said the declaration of a "default event" may force the euro zone to act on debt relief for Greece in any new negotiations after Sunday's referendum, although euro zone leaders have long sought to delay this option.

"This means a public debt cut for Greece must come much faster than previously expected," said Marcel Fratzscher, head of the Berlin-based DIW economic institute. The default meant the euro zone needed "to first restore Greece’s debt sustainability before agreeing on a new aid program".

The EFSF said its would decide on future actions in consultation with euro zone states, the European Commission and the IMF.

The Greek non-payment had no influence on the EFSF's capacity to repay its bondholders, the statement said, underlining that the fund relies on a "robust guarantee structure" to borrow on capital markets.

The board of directors of the EFSF is composed of deputy finance ministers and senior officials of euro zone states.

Incorporated in June 2010, the EFSF issues bonds to provide loans to countries in financial difficulties. From July 2013 it was subsumed into the euro zone's permanent bailout fund, the European Stability Mechanism (ESM), also based in Luxembourg and headed by Regling.

(Reporting by Francesco Guarascio; Editing by Paul Taylor and Kevin Liffey)