Long-distance runner Jake Smith has switched allegiance to England - six weeks after breaking the Welsh half-marathon record.
Bermuda-born Smith, 22, was eligible for Wales having been based in Cardiff, where he studied at Cardiff Met, for three years.
He broke Steve Jones' 34-year-old Welsh record at October's World Half Marathon Championships.
"This is a hard decision for me to make," he said.
"But I feel it's the right decision.
"I thought at first it was only natural for me to turn Welsh as I live here and have done for the past three years but I feel as though I hadn't thought it through properly."
Smith ran one hour 36 seconds to finish 18th in Poland - his first senior race for Great Britain - the third fastest time ever recorded by a British athlete over the distance after Mo Farah and Callum Hawkins.
"The big turning point for me was when I broke Steve Jones' - a Welsh legend of distance running - Half Marathon record after only being Welsh for four days," Smith wrote on Instagram.
"As both my parents were born in England and I don't have any family ties in Wales, I would feel much better representing England at future competitions.
"However, I would like to thank Welsh Athletics for everything they have done for me as their support to date has been incredible."