The phoney letter, attached here, asks for confidential banking information to be faxed to the independent government office, which handles major government contracts.
It is thought many smaller architectural practices and contractors may have been lured into giving away their financial details to the conmen, hoping to join preferred procurement lists held by the Treasury organisation.
An OGC spokesman confirmed they had already received hundreds of calls about the fake requests, which are headed with convincing addresses and contact details.
The police have begun a full investigation into the matter, although the OGC was unaware of anybody having fallen for the scam ( Architects warned of scam threat).
The OGC spokesman said: 'The letters ask people to respond by fax and this is clearly not how we do business.
'However, there may have been smaller organisations wanting to get on the government list which may have been tempted to reply.'
Anyone with concerns should contact the OGC Service Desk on 0845 000 4999.