"They cannot extend their neck, so they cannot fish."

While captured animals can usually be saved, and are treated with antibiotics for months before being released, some are not so lucky.

Ms Charles said that a young seal had to be put to sleep recently when a nylon tie severed its nose.

Plastic can often cut through several inches of blubber on a trapped seal's neck, causing painful cuts and infections.

Because they are shy creatures and difficult to approach, they often have to be slowed by infection before they can be helped.

Disturbances to the group can also cause long-term problems for the population; pregnant mothers miscarriage when stressed.

Rangers at the famous National Trust seal-spotting haven Blakeney Point in North Norfolk face a constant battle against plastic.

A spokesperson for the National Trust told the Telegraph: "We carry out regular beach cleans on the Norfolk coast, we are just coming into pupping season at Blakeney Point, we had our first pup born this week.

"Wardens cleaned the beaches to make sure they were as clean as possible We do often have occasions where we have to take a seal to the RSPCA because of plastic, it does happen.

"We have rangers, volunteers and members of the public. More people are becoming aware of that sort of pollution and do their beach cleans across Norfolk.