FORDOUN, Scotland — The first inkling that Ross Mitchell knew something had gone awry was when a busload of Bulgarians he had hired to pick blueberries last month failed to appear.

The way he saw it, they had been “hijacked.”

The real explanation was rather more prosaic but no more palatable: During a one-night stopover in Birmingham, a city on the way to Mr. Mitchell’s farm in northern Scotland, the 30 Bulgarians were lured by a factory offering more attractive wages. They never made it to their original destination.

The diversion of his work force meant that Mr. Mitchell, a fruit supplier for a major supermarket chain in Britain, lost 50 tons of fruit worth half a million pounds ($680,000) in a matter of weeks.

Mr. Mitchell is not alone in his distress. Similar anecdotes have been reported by a wide range of employers, notably the National Health Service and the hospitality sector. It is a phenomenon tied to Britain’s decision last year to leave the European Union, a process known as Brexit. Since then, thousands of Europeans have already left Britain or have decided not to return, causing a significant drop in migration to the country. Net migration in the year ending in June fell by 106,000, about a third, the government said, “the largest fall in any 12-month period since records began in 1964.”