The move will raise eyebrows among observers scrutinising the progress of the competition, which will see six new gateway projects built at strategic sites around Wales (Landmark Wales competition shortlist revealed ).
McChesney, who was named on a five-strong shortlist for the Flintshire Border, has been replaced by 'first reserve', Graeme Massie Architects from Edinburgh.
The AJ 40 Under 40 star will now miss out on a £5,000 honorarium to develop his designs.
Speaking about his decision to leave the contest, McChesney said: 'As well as the Landmark Wales contest, we were simultaneously invited on to shortlists for two RIBA-organised competitions: Middlesbrough A66 and New Islington Footbridge.
'In addition, our new pavilion for Avenham Park in Preston has just started on site, so we suddenly found ourselves over-committed.
He added: 'We had to make careful decisions about where to place our efforts and in the end decided to focus on the projects with guaranteed funding expectations.
A spokesman for the competition organisers said they were 'sorry to lose' the practice from the contest.