Under changes introduced in July as part of efforts to reduce migration, individuals must earn at least £41,700 to obtain a skilled worker visa, up from £38,700.
The starting salary for a prison officer, external is generally below this amount, particularly outside London.
The Prison Officers Association (POA) said the salary threshold changes meant jails stood to lose more than 2,500 overseas recruits, which it said would have had “a catastrophic effect on prison stability”.
Meanwhile, chief inspector of prisons Charlie Taylor warned earlier this year that many jails were heavily dependent on prison officers recruited from west Africa.
He said many were in danger of not having their visas renewed, which would have a “devastating effect” on prisons.
The exemption only applies to applicants already in the country and lasts until the end of 2026, with a lower salary threshold of £33,400 in place until 31 December 2027.
The POA welcomed the new exemption, calling it a victory for “common sense”.
The trade union’s general secretary Steve Gillan said: “It might not be perfect, but it will mean the prison service hopefully can remain stable.”
The association’s national chairman Mark Fairhurst added: “Our members can now go about their daily lives without the threat of removal from the country.”