Melbourne Cup entrant Kew Gardens has lowered the colours of the previously unbeaten Lah Ti Dar Tin the St Leger at Doncaster.

Kew Gardens' victory on Saturday (Sunday AEST) gave champion trainer Aidan O'Brien his sixth victory in the world's oldest Classic.

Having made a spectacular return at York last month after an enforced summer absence, Lah Ti Dar was earlier this week given the green light to take on her male counterparts and was sent out the market leader.

Her half-brother Too Darn Hot had won the Champagne Stakes earlier on the program and Lah Ti Dar was all the rage to give trainer John Gosden, jockey Frankie Dettori and owner-breeders Lord and Lady Lloyd Webber a big-race double.

But Kew Gardens had not read the script.

The 3-1 chance travelled powerfully midfield under Ryan Moore, with 7-4 favourite Lah Ti Dar always in sight.

He passed her and went on to win by two lengths.

"We couldn't be happier with him," O'Brien said.

"We thought he was a horse that would stay well and we thought the distance (2900m) would suit him."

Kew Gardens is on the third line of the TAB's Melbourne Cup market at $17 and is now as short as 12-1 with some British bookmakers for the Prix de l'Arc de Triomphe.