The 2.5cm-wide solid plastic ring – believed to have been debris from pipework – left a 7cm-deep wound in the seal’s neck which had become infected.
The staff at the RSPCA’s East Winch Wildlife Centre near King’s Lynn, which carried out the rescue and subsequent rehabilitation, affectionately named the seal “Mrs Vicar”, due to the somewhat clerical appearance the ring had given her.
The charity said it was the worst injury of its type it had seen.
The adult grey seal was captured at Horsey Beach in Norfolk on 4 April this year after first being sighted with the ring stuck around her neck more than two years previously.
Volunteer rescuers from Friends of Horsey Seals caught her and a vet then cut away the plastic.
To help the wound heal Mrs Vicar was given salty baths and after three months at the centre she was strong enough to return to the wild.
She was released into the River Nene at Sutton Bridge in Lincolnshire on Tuesday, so she can make her way back to the North Sea.
Ben Kirby from RSPCA East Winch, said: “The wound on Mrs Vicar’s neck will always be very visible and is a sad reminder of how much damage plastics in our seas can cause to the natural world.
“The vets and staff at East Winch have worked incredibly hard to rehabilitate this special seal so that she could one day be returned back to the wild.
“While the scars around her neck will always remain, the salt water will continue to heal her neck.
“She’s now a healthy weight and we’ve done all that we can for her – it’s now up to her.
“Since day one she has just fought and fought to survive and never gave up, despite how sick she was.
“We just all feel so proud to have been able to help her with this second chance.”
Alison Charles, former centre manager at RSPCA East Winch who has since retired, previously said: “When I first saw how severe Mrs Vicar’s wounds were I was really worried she wouldn’t be able to make it.
“It was just so severe and infected and you could smell the infected flesh, it was just awful.
“When the ring was removed it then meant that her body released a huge swell of dangerous toxins, which she then had to fight off.
“So for the first few days she didn’t really move or show any signs of improvement - and although this is something we do see with necklace-injured seals, it was still very worrying that she wasn’t going to pull through.
“However each day there was a small sign of improvement and she started eating and her salt baths began to work on the infected wound.”
Several weeks into her recovery Mrs Vicar was moved to an outside pool where staff helped her build up her strength by getting her to swim from one end to the other for fish.
At least three seals have had similar discs caught around their necks in recent years.
Additional reporting by PA.