The 500lb British-made mine was discovered nearly 20 years ago, in 1993, and threatens the Far North Liquids and Associated Gas System (Flags) pipeline.
Flags brings in 5% of the UK's daily gas supply.
Shell announced in June that it would call in a subsea bomb squad for the mine, which lies 62 miles off St Fergus on the Aberdeenshire coast.
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The company said the operation was now under way after risk assessments and approval.
A company spokesperson said: "Our priority is working to ensure a safe outcome, and we will provide more information once we have completed the operation."
Since a study into the mine last year, a guard ship has warded off fishing boats in the area.
A spokeswoman said in June that although Shell had previously followed military advice to leave the mine "in position" and monitor it, new technology meant the company could now contemplate detonating it.
It had planned to use Remote Explosive Ordnance Disposal System technology, which consists of a bag that will inflate around the mine and float to the surface.
The company liaised with the Department of Energy and Climate Change, the Royal Navy, Grampian Police and the Health and Safety Executive prior to taking on the delicate manoeuvre.
(NE)
