Boscombe Pier in Dorset is undergoing emergency repairs after it was discovered that structural piles supporting the end had become exposed.
Bournemouth Borough Council said an "unexpected change" in the sea bed level had caused the problem.
Contractors arrived on site on Thursday to begin four weeks of unscheduled works to protect the supports.
The end section of the pier, which was built in 2008, will be closed to the public while the work is carried out.
The council said "small sections" of the piles at the end of the pier were exposed, meaning they were "now in need of some additional protection".
Contractor Carillion has already begun moving equipment to the site, which includes a mini crane to lift divers in and out of the water.
The council said the pier will remain open but there will be a 12 to 15ft (3.6 to 4.5m) fenced off area stretching across the end.
Leisure and tourism councillor Anne Filer said: "These works are essential in maintaining Boscombe Pier which has been a key part of the overall regeneration of Boscombe seafront."
Carillion project manager Greg Smith said: "Carillion will complete these works as quickly as possible and in order to do so asks the public to keep clear of the working area on the pier head during the construction period."
The refurbishment of the pier and construction of a nearby surf reef were part of an £11m regeneration project to revive Boscombe seafront.