Trainer Ed Vaughan cited reduced prize money as he revealed plans to give up his British licence.
The surprise news came just a week after the Irishman saddled Dame Malliot, who gave jockey Hollie Doyle her first Group victory.
"I haven't just woken up and decided to do this," said the Newmarket trainer, who has held a licence since 2004.
"It's been on my mind for a while and now seems the right time to finish up training here in the UK."
Vaughan, whose big winners included Robin Hoods Bay in the Winter Derby at Lingfield in 2014, aims to stay in the racing industry and has not ruled out training elsewhere.
Prize money has been a point of contention in British horse racing for some time, but the issue has intensified because of financial restrictions caused by the coronavirus pandemic.
"I think things are going to get very tough when we are already in the situation where we have prize money that hardly covers the cost of keeping a horse in training for a month," he said.
"As everyone is aware, with the reductions in prize money and the cost of running a business being so high, the economics of training in Britain are not good. I'm taking this decision now because I can see things getting worse in the next year."
Frank Keogh, BBC Sport
When a respected and popular trainer who has just saddled a Group winner says he cannot make ends meet, you know it is time to take notice.
Dame Malliot earned prize money of £32,000 for winning at Newmarket, while £56,000 went to the victor last year.
And Vaughan's warning that things will get worse means he is unlikely to be the last trainer to quit operating in Britain.
Meanwhile, many in racing will hope he can land a top-level Group One victory with Dame Malliot before the season is finished.