His final season was disrupted by injury problems, but he ended his career with his biggest success when he became the first British-trained horse to win the Japan Cup.

Jupiter Island was a "close-coupled, quite attractive"[2] but "rather narrow"[3] horse with a white coronet on his left hind foot, bred by the Marquess of Tavistock.

Brittain has been regarded as a pioneer of intercontinental racing, sending Bold Arrangement to finish second in the Kentucky Derby and winning the Breeders' Cup Turf with the filly Pebbles.

[7] Jupiter Island was beaten in his first three races as a four-year-old before winning the Air New Zealand Handicap over one and a half mile at Newmarket Racecourse, beating Keelby Kavalier by a neck.

After winning another handicap at Newmarket he was moved up in class to contest the Group Three St Simon Stakes at Newbury on 22 October by which time he was being described as "one of the most improved horses in training" in the British press.

When moved up to the highest level he finished sixth to Teenoso in the Group One King George VI and Queen Elizabeth Stakes at Ascot in July.

[3] On his six-year-old debut, Jupiter Island, ridden by Greville Starkey won the Group Three John Porter Stakes at Newbury, beating Ilium and Gay Lemur at odds of 11/2.

Ridden by Tony Ives, Jupiter Island accelerated clear of his opponents in the straight and won convincingly by three lengths from Baby Turk and Saint Estephe.

Jupiter Island, who had been flown out two weeks before the race to give him an opportunity to acclimatise to the local conditions, was ridden by Pat Eddery and started at odds of 13.9/1 in front of a crowd of almost 100,000.

[12] Jupiter Island was in last place on the turn into the straight,[13] but produced a strong late run to catch the British-trained three-year-old Allez Milord in the final strides and prevailed by a head after a "desperate finish".

The result was only confirmed after an enquiry by racecourse stewards into possible interference by the winner in the closing stages and an attempted protest by Greville Starkey, the rider of the runner-up.

He was not included in the official International Classification, but in the British Free Handicap he was rated sixteen pounds inferior to the top-rated older horse Diamond Shoal.