Iran's parliament has dropped a summons calling President Mahmoud Ahmadinejad in for questioning, Iranian media reported on Wednesday, signalling an uneasy truce after months of political struggle between rival conservative factions.

One hundred of parliament's 290 members signed a motion in June summoning Ahmadinejad to face questions, amid mounting criticism, particularly from hardline conservatives who accuse the president of riding roughshod over the legislature.

But, with Supreme Leader Ayatollah Ali Khamenei calling for unity among the branches of power, parliament's presiding board held back from issuing the summons and Speaker Ali Larijani said it was now invalid as several lawmakers had pulled out.

Sixty-nine signatures remain ... others have been withdrawn, Larijani told state radio. That is less than the 75 signatures needed to force the president to face parliament.

The truce comes as Iran faces increasing pressure from Washington, which wants to tighten sanctions on Iran over its nuclear programme after it discovered what it says was an Iranian plot to kill the Saudi ambassador to the United States. Iran denies it was involved in any such scheme.

Top-level policy in the most sensitive areas is set by the supreme leader so the position of the president on issues like the nuclear programme and dealing with the West is not decisive.

Many lawmakers are unhappy at parliament's failure to call Ahmadinejad to order and one, Ali Motahari, resigned in protest.

This is a limitation on the authority of lawmakers, stopping them doing part of their jobs, the semi-official Fars news agency quoted Motahari as saying.

Parliament, often at odds with Ahmadinejad on policy issues, increased its pressure on Ahmadinejad after he attempted in April to sack the intelligence minister, who plays a key role in overseeing elections. That move was blocked by a rare intervention by Khamenei.

Hardliners accuse Ahmadinejad of being in the thrall of a deviant current of advisers seeking to undermine the authority of the clergy in the Islamic Republic's system of government.

Analysts say Khamenei may be happy to have a weakened Ahmadinejad but does not want to risk the political upheaval of him being forced out of office with less than two years of his term left.

Khamenei has suggested that in future Iran could scrap the position of a directly elected presidency and have the head of government elected by parliament -- something which critics say could weaken Iran's version of democracy.

A $2.6 billion (1.6 billion pound) bank fraud has given additional ammunition to Ahmadinejad's critics, as some politicians have linked the main suspect to the so-called deviant current and parliament has launched an impeachment procedure against the economy minister.

(Reporting By Mitra Amiri; Editing by Jon Boyle)