(adds quotes on Hezbollah, Saudi advisory, background)

By Samia Nakhoul

LONDON, Feb 19 (Reuters) - Next month's Arab summit in Syria will collapse if a solution to Lebanon's political crisis cannot be found by then, Lebanon's Prime Minister Fouad Siniora said on Tuesday.

The election of a new Lebanese president has been obstructed since November and Siniora told Reuters in an interview efforts were being made to fill the post and prevent a power vacuum, after the worst street clashes since the 1975-90 civil war.

Siniora, whose anti-Syrian ruling coalition is locked in a 15-month power struggle against an opposition led by Shi'ite Hezbollah, which is backed by Syria and Iran, said that without a Lebanese president -- and the possibility that other Arab leaders could boycott the gathering in solidarity -- the summit would lose its value.

"I believe that the lack of representation of Lebanon on a presidential level at the summit will make the summit lose a lot of its importance," he said.

"This summit should be attended by all the presidents and all the Arab countries," said Siniora, who was in Britain for talks with Prime Minister Gordon Brown.

Diplomats say Saudi Arabia's King Abdullah is unlikely to attend the Arab League's annual summit unless Lebanon's political deadlock is resolved.

SUMMIT BOYCOTT?

Asked whether Arab states, including Saudi Arabia, might not send top-level representation or even boycott the summit, Siniora said no decisions had been reached.

"I don't really have any information that confirms the position of every Arab country. Lebanon has not asked any Arab country to boycott," he said.

"We are saying that Lebanon should be represented by a president and all the Arabs expect Lebanon to be represented on a presidential level," he said about the summit due to be held in Damascus at the end of March.

"This is what we are seeking and what others are seeking to facilitate: the election of the president and to remove all the hurdles that are being placed from inside and outside Lebanon."

Saudi Arabia has emerged as a leading Arab power in recent years, as surging world oil prices have enabled the U.S. ally to play a more forceful role in settling regional disputes.

Riyadh has thrown its weight behind the Siniora government and mediated between Beirut and Damascus.

But as tensions have risen between Lebanon and Syria it recently warned its citizens not to travel to Lebanon because of deteriorating security and following strains in ties between both countries over Lebanon.

France, another ally of Siniora's, has just closed two of its cultural centres in Lebanon for the same reason.

The prime minister said both moves were triggered by the escalation in rhetoric inside Lebanon, which led to unrest in the streets.

He also said that a threat by Hezbollah leader Hassan Nasrallah to spread the conflict with Israel into "open war" had raised the national and regional temperature.

"The statement by Nasrallah does not serve the interests of Lebanon, Hezbollah or the Islamic cause...," he said.

He said Lebanon has already paid a high price for attacks carried out by Hezbollah against U.S. and Western targets, implicitly referring to Imad Moughniyah, the Hezbollah military commander who was killed last week in a car bomb in Damascus.

Hezbollah and its main backer Iran accused Israel of assassinating him. Israel rejected the charge, though its Mossad spy service had long sought to kill him.

Moughniyah was implicated in the 1983 bombings of the U.S. embassy and U.S. Marine and French peacekeeping forces in Beirut, the 1985 hijacking of a TWA airliner as well as the kidnapping of Westerners in Lebanon in the 1980s.

Israel accuses Moughniyah of planning the 1994 bombing of a Jewish centre in Buenos Aires and involvement in the 1992 bombing of the Israeli embassy in the Argentine capital.

Lebanon's 15-year civil war ended in 1990 with a Saudi-brokered peace pact which has been strained to breaking point since the assassination in 2005 of former Prime Minister Rafik al-Hariri.

The governing coalition accuses Syria of killing Hariri and other anti-Syrian figures assassinated since his death. Syria denies any involvement.

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