Graeme Russell, the former chief of Manchester's troubled architecture centre CUBE, is understood to have handed in his notice in the last 48 hours.
Portland Place officials claim Russell, who was head of programmes for the RIBA Trust, will be retained in some consultancy capacity from a base in the North West.
But Russell's departure from London will come as a hammer blow to the RIBA Trust, which said his appointment had marked a new start for architecture exhibitions in the UK.
It will certainly put the brakes on the trust's ambitious plans for a heavily expanded programme of exhibition and outreach events.
The reasons behind Russell's decision to walk away are as yet unclear but it is understood that the curator - whose family remained behind in Manchester when he moved to London - was having trouble settling in the capital.