In June, Marwell was matched against colts and older horses at Royal Ascot in the King's Stand Stakes, one of three Group One, all-aged sprints run in Europe at that time. Starting the 5/4 favourite, she overtook the front-running Standaan approaching the final furlong and pulled clear with what was described as "a blistering turn of acceleration" to win impressively. On her next appearance, she ran in the six_furlong July Cup at Newmarket, in which she was opposed by Moorestyle, the outstanding sprinter of 1980. Swinburn had to attend a disciplinary hearing at the Jockey Club on the morning of the race but had a car standing by and reached Newmarket in time to ride the filly. Marwell and Moorestyle moved to the front of the field a furlong from the finish before the filly accelerated clear to win. Marwell returned to five furlongs for the Group Two William Hill Sprint Championship on soft ground at York Racecourse in August. In what was considered a major upset, she finished second, beaten two and a half lengths by Sharpo, who recorded the second of his three wins in the race at odds of 14/1.
