Bellutti, who had beaten McGregor in the first round and set a new Olympic record of 3min 34.130sec in the process, was again too powerful for the world hour record-holder.
The Italian was 2.188sec up on McGregor after the first lap and although the former triathlete cut that deficit back to 1.141 sec by lap five, Bellutti then moved up a gear. By the end of the race, Bellutti, who had improved her own Olympic record to 3:32.371 yesterday, was in the same stretch of track as the British rider.
The Italian finished in 3:34.404 to book a final meeting with France's Marion Clignet, who defeated German teenager Judith Arndt in the first semi-final.
But the 20-year-old had the consolation of the bronze medal, her time of 3:38.744 pipping McGregor for third place, as the Yorkshirewoman registered 3:40.885.
But if British morale was low, they could at least take some consolation from America's failure. The USA pumped over $1m into Project 96 to develop a superbike. It was intended to reverse the Americans' poor run of results, but so far they have only one silver medal.
Made of razor-thin carbon fibre, the superbike weighed in at 16lb, ready to take on the world. However, no one appeared to have informed the Russians, who raced on heavier bikes and qualified fastest in the team pursuit as the USA went out.
Another for whom failure was painful was the Australian Shane Kelly. At the start of the 1,000m time-trial he was two minutes away from adding Olympic gold to a world title.Then his foot slipped off the pedal and four years' work was wasted.Reuse content