The causeway at Carew Castle, which forms part of a mile-long walk around the Millpond, will be completely closed for around three weeks initially, with further closures possible as work progresses.
National Park Authority Building Projects Officer, Andrew Muskett said: “Much of the essential work will focus on the sluice area and will include work to help prevent water leaking through the structure and repairs to the sluice gates.
“While the work will be completed as soon as possible, it is difficult to give exact dates at this time due to the tidal conditions and the fact the work is below ground.”
The Grade II* listed causeway at Carew Castle and Tidal Mill is part of the only remaining Tidal Mill and Causeway in Wales, dating back to 1541.
The National Park Authority manages Carew Castle, Mill and causeway on a long-term lease from the Carew Estate.
The work will be funded by the Welsh Government.
(Lead image: Pembrokeshire Coast National Park)