EnQuest took the decision to evacuate all 115 workers from its Thistle Alpha platform in the UK northern North Sea following an inspection on a “support element on a redundant subsea storage tank”, the operator said on Tuesday afternoon.

The update adds more detail to what was behind the emergency downmanning that began on Monday night.

All the workers, who were flown to the nearby Dunlin Alpha facility, are now being transferred to shore, EnQuest said.

Bob Davenport, EnQuest’s North Sea managing director, said Thistle will remain shut in while further inspection work is carried out and “any necessary remedial action undertaken”.

Davenport said: “The safety of our people is our absolute priority. Our offshore installation manager took proactive action to transfer everyone from the platform as a precaution following yesterday’s inspection.

“This was carried out safely and quickly, with plans then made for their onward travel home.

“I’d like to thank everyone involved for their support including the team on board Dunlin, employees, contractors and the Maritime & Coastguard Agency.”

EnQuest said earlier "proactively downmanned" Thistle following a "preliminary subsea structural inspection".

The UK's Coastguard was informed at about 6.30pm local time on Monday.

A UK coastguard helicopter from Sumburgh in Shetland was assisted by two other aircraft from Norway to transfer the personnel to the nearby Dunlin Alpha platform.

"The platform has been safely shut down in accordance with planned procedures," an EnQuest spokesperson said earlier.

The Thistle platform is located about 200 kilometres north-east of Shetland in Block 211/18.

The Health & Safety Executive (HSE), the UK safety regulator, said it was aware of the evacuation and is making further inquiries.