The unanimous view among Government and police officials was that the firing of mortars with delayed or remote-control mechanisms -- a method used by the I.R.A. in scores of attacks on police stations in Northern Ireland -- ruled out the likelihood that the attack was carried out by a Middle East terrorist organization. But the I.R.A. had not used mortars outside of Northern Ireland before.

Some officials said they were appalled that such an attack could be launched from a prime tourist site only 150 yards from the Prime Minister's office.

The van was abandoned at the intersection of Horse Guards Avenue and Whitehall despite a heightened terrorist alert in Central London because of the war in the Persian Gulf. Government and police officials implied it had been there only a few moments, but a former Home Secretary, Merlyn Rees, maintained in the House of Commons this afternoon that it might have been there for eight minutes before the mortars shot off. Snow Deters Tourists

The terrorist alert because of the war in the gulf continues at airports, railroad stations, schools and office buildings throughout London. This morning's attack was a reminder that Britain has also been involved with other unrest, that in Northern Ireland, for two decades.

On a good day, the Horse Guards on Whitehall, opposite the spot from which the mortars were fired, would have been thronged by scores of tourists. But the snow and bitter cold kept most of them off the streets.

"There is no doubt in my mind that this is the work of Irish Republican terrorists and there are no links with any Arab terrorist groups," said Comdr. George Churchill-Coleman, the head of Scotland Yard's antiterrorist squad. He appealed for information about the movements of the van, which had the registration number A862 NAR and had been bought in London last July by three men who paid cash. Van Bursts Into Flames

Commander Churchill-Coleman said that one man had been seen abandoning the van and running to an accomplice waiting on a motorcycle. The van burst into flames after the mortars were fired, and it has not yet been determined if the fire was deliberately set.