LISBON, Oct 3 (Reuters) - Portuguese prosecutors said on Saturday they had opened a probe into the legality of the centre-right government's authorisation of the United States' use of the Lajes Air Base in the Azores during the Iran conflict, following a complaint by an opposition party.

A longstanding agreement with the US allows Washington to use the base without prior authorisation during peacetime, but requires Portugal to give approval once hostilities begin.

Portugal, unlike neighbouring Spain, authorised several US transit flights through the Lajes air base, a decision that drew criticism from left-wing opposition parties and triggered a political debate over Lisbon's role in operations linked to the conflict with Iran.

In a statement, the Prosecutor General's Office said "the probe was opened following a complaint filed by members of the Left Bloc party," without providing further details.

Portugal's Foreign Ministry declined to comment.

The Left Bloc asked prosecutors in April to investigate the US use of the Lajes base, arguing it amounted to a "gross violation of international law."

However, Foreign Minister Paulo Rangel has said Portugal was not involved in the conflict and that the use of the Lajes base complied with international law, having been authorised only after assurances from Washington that any operation would be defensive, necessary and proportionate, and limited to military targets.

Portugal's prosecutors are bound by the principle of legality, requiring them to investigate alleged wrongdoing when there are sufficient grounds to do so, unlike in some jurisdictions where prosecutors have broader discretion to decide whether a case should proceed.

The Lajes base, located in Portugal's Atlantic Azores archipelago, is a strategic transit hub for US forces under a bilateral defence treaty dating back to 1951 and hosts the 65th Air Base Wing of the US Air Force, providing support for US, NATO and allied operations.

(Reporting by Sergio Goncalves; Editing by Toby Chopra)