Seals at Lincolnshire's Donna Nook Nature Reserve are being put at risk by visitors who ignore warnings not to get too close.
Lincolnshire Wildlife Trust says hundreds of people are refusing to stay in the viewing area and walking on the beach to take photographs.
Thousands of people travel to Donna Nook each year to witness the 1,300 seal pups born near the sand dunes.
The trust warns that human contact can lead to mother seals abandoning pups.
Seals can also become aggressive and are capable of biting.
For most of the year grey seals at Donna Nook are at sea or out of sight on distant sandbanks, but every November and December the seals come inland to give birth to their pups.
'Very sharp teeth'
Although the beach is open to the public, visitors are asked to keep to a viewing area at the foot of the sand dunes to reduce disturbance to the seals.
Paul Learoyd, chief executive of Lincolnshire Wildlife Trust, said: "In the past we've had problems with animals biting people.
"They look very cute but they do come with very sharp teeth.
"People will disturb these animals. Some of these pups have just been born, they're feeding from their mother, and it's important that that sort of behaviour isn't disturbed."
This year the trust has increased the number of wardens patrolling the site to 50 and has also improved signage on the site in the hope of encouraging people to stay off the beach.