The man was working inside the trench in the village of Letterston when the sides caved in. A specialist rescue crew is at the scene, along with firemen and paramedics.
More top news
Wales's best known hotel is under fire for advertising jobs paying only £5 an hour.
The tide is turning in favour of more unsettled weather over the next few days!
The world record attempt will raise money for the foundation set up in memory of the Olympic gold medallist, who died in May 2013.