King Juan Carlos was reported today to be determined to go ahead with a planned weekend visit to Barcelona, where he is to review a key military parade, despite evidence of a neofascist plot in the already troubled city.

Police in Barcelona reported the discovery of a tunnel close to the parade route. The tunnel led from a ground-floor office that had been rented last month by a terrorist killed on Sunday when commandos freed hostages from a main city bank.

The bank seizure was linked to the military coup attempt last February which failed when Juan Carlos called on the armed forces to support the constitutional order.

Palace spokesmen said no change of plans in the royal trip was envisaged. The annual armed forces day parade is to be held for the first time in Barcelona, and Juan Carlos, who is commander-in-chief of the armed forces, is due to take the salute from a tribune on the main boulevard. The scheduled motorcade passes within 30 yards of the end of the still incomplete tunnel, which was said to be within 16 feet of a sewer that crossed the parade route.

In 1973, a massive explosive charge planted by terrorists in a tunnel killed then prime minister Luis Carrero Blanco, considered the political heir to dictator Francisco Franco.

The parade had already taken on added significance in the wake of the February putsch and is projected as a major public relations effort to bring the people and the services together.

Reliable sources said the 43-year-old monarch might face public hostility from rightists in Barcelona during his visit. Tension remained at a peak since last weekend's seizure of a main Barcelona bank by petty criminals who attempted to barter the lives of more than 200 hostages for the freedom of four officers indicted in the February coup attempt.

Juan Carlos' role during the putsch earned him the enduring hatred of extreme rightists, who have promoted the coup leaders to the status of heroes.

Strengthening the conspiracy belief was the discovery in the rented office, where the tunnel started, of 39 false identity cards along with several handguns, a small amount of explosives and a fuse cord. Police also found a newspaper dated Sunday, indicating that work on the tunnel was in progress at the height of the bank seizure.

Premier Leopoldo Calvo-Sotelo told Congress last night the government believed that the bank raid was mastermined by extreme right-wingers but that he was unable to say who they were. Opposition leader Felipe Gonzalez, the chief of the Socialist Party, said: "We are faced with a perfectly organized harassment of our democratic state."