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.
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Tonight will remain mostly dry apart from the odd shower in places.
The mother of missing 19-year-old James Corfield from Montgomery read an emotional statement at the Royal Welsh showground this afternoon.
71-year-old Raymond Hodges was convicted of 24 charges spanning almost a decade.