JERUSALEM - An answer to the biggest remaining mystery of the 1973 Yom Kippur War between Israel and much of the Arab world - who warned Israel of the impending attack at almost the last moment? - has now been offered. An Israeli historian, Dr Ahron (Ronnie) Bregman, who lives in England has identified the agent as none other than the son-in-law of former Egyptian president Gamal Abdel Nasser. Although Nasser died in 1970, his son-in-law Ashraf Marwan was part of the inner circle of his successor, Anwar Sadat, who launched the war.

The identity of the agent, described by a postwar Israeli inquiry commission only as 'the source', has been a closely guarded secret, although the available evidence pointed towards someone high in the Egyptian establishment.

From published accounts based on Israeli sources, it is known that he was a 'walk-in' who entered an Israeli embassy in a European capital and offered his services in 1969. Extensive checks convinced Mossad that he was not a double-agent.

In the ensuing years, he provided information on Egypt and the Arab world that Israeli Defence Minister Moshe Dayan and others would later term priceless.

On the evening of Thursday, Oct 4, 1973 - two days before Yom Kippur - he telephoned his Mossad handler in Europe and asked for an urgent meeting with Mossad head Zvi Zamir. He also let drop a codeword for imminent war.

Mr Zamir flew to Europe and met him the next night. The agent informed him that Egypt and Syria would attack the next day at 6 pm.

That had indeed been the original hour planned but, apparently unknown to him, it had been changed two days before to 2 pm. It was not until about 4 am on Yom Kippur that Prime Minister Golda Meir and her military chiefs received the information.

Although only 10 hours remained before the attack, the warning was precious. It permitted the mobilisation process to get under way and the military and political hierarchy to begin to adjust their mindsets to the stark realities before the Arab attack began.

Had it not been for the warning, the Golan Heights would almost certainly have fallen and the situation on the Egyptian front would have been even more grievous than it was. The Israeli army was on the road to Cairo and Damascus when the war stopped.

The head of Israeli military intelligence, General Eli Zeira, would later contend that 'the source' was in fact a double agent who had lulled Israel into believing Egypt would not go to war and who had delivered the October warning only at the last minute.

Others in the intelligence community argued that Gen Zeira, who was dismissed at the recommendation of the inquiry commission, was trying to put the blame on a Mossad agent as a way of exonerating military intelligence - and particularly himself - for insisting that the chances of war were low despite abundant evidence that the Arabs were preparing for war.

In his book "A History of Israel" and in an article he wrote for an Israeli newspaper a few months ago, historian Ahron Bregman referred to the agent as 'the in-law' and said he had family connections to Nasser.

The article reached the attention of the Egyptian media, which assumed the reference to be to Mr Marwan, who is married to Nasser's daughter, Mona. It was a breath-taking allegation since Mr Marwan, 58, remains one of the best connected people in the Egyptian establishment.

When interviewed about the allegation last month by the Egyptian newspaper Saut el-Uma, Mr Marwan dismissed it as 'an absurd detective story.'