Repairs are being carried out to part of the Headland’s ancient Town Wall after suffering damage by recent strong seas.
Powerful waves have battered part of the defensive structure, which dates back to the 14th Century, dislodging a number of stones at its base.
A specialist contractor has been out on site to begin repairs under an emergency license.
The surviving section of the medieval wall, including the Sandwell Gate, is protected as a Scheduled Monument and as a Grade One Listed Building.
Greg Purcell, 57, who lives in a house on the adjacent road called Town Wall, noticed the damage last Tuesday and reported it to Hartlepool Borough Council and managers of the nearby fish quay PD Ports.
Mr Purcell said: “The sea had been pretty heavy for the last week or so. I get a really good view of that bit of the beach and could see more stone than usual on the beach and when I looked there was a big gap at the base of the wall.
“It was about three or four feet deep and about 15 feet wide.
“Someone from PD Ports came out to look at it and said it is going to be a big job to fix.”
He said it is close to an area of the wall that collapsed 51 years ago.
“Last time it happened there was a real danger of houses falling into the hole,” he said.
A Hartlepool council spokesman said they were aware of the damage.
They said: “PD Ports is also aware of the damage, which has been caused by recent high tide events, and sympathetic work to repair the wall commenced on Thursday.
“The work will be carried out by a specialist contractor under an emergency licence.
“The level of damage demonstrates the importance of Hartlepool Council’s continuing flood and coastal defence work to protect our residents, including the recently completed set back wall.”