The young pup called Gnocchi was rescued and taken to the RSPCA’s wildlife centre in East Winch in Norfolk.
Born last year, it had a bad injury caused by a plastic bag embedded around his neck.
Alison Charles, centre manager, said: “My heart actually sank when I realised this poor seal was suffering as a result of someone’s discarded litter.
“You really think you’ve seen it all and then along comes something new - which inflicts horrific injuries and causes so much damage.
“It seems such an innocuous object - but this small plastic bag has caused untold pain and injury for poor Gnocchi and it makes me so cross because it could have easily been avoided if people disposed of their litter properly.
''We have to do more to stop rubbish ending up in our waters and causing such damage to our precious wildlife.
“Gnocchi is going to need weeks, if not months of care, his injury is very deep and infected.
“We’ve removed all of the bag from his neck and he’s currently being given pain relief and antibiotics as well as regular salt baths.
“The trouble with these types of injuries is that the seals get something caught around their neck - but they can’t get them off by themselves and so become trapped in them.
“And as they grow bigger the object then cuts deeper and deeper into their neck - causing these horrendous ‘necklace’ injuries which are sadly becoming more and more common in seals.”
The RSPCA East Winch Wildlife Centre has launched a fundraiser to help pay for fish to feed the sick, orphaned and injured seals in their care.
The centre cares for over 150 seals each year and the cost of mackerel to feed them is rising, costing the charity a whopping £3,999 for three pallets of mackerel.
You can help support the wildlife centre and provide fish and salt for seals in need by donating here.