Senior figures at the RIBA, CABE, the Royal Town Planning Institute, the Royal Institute of Chartered Surveyors and the National Trust are in talks about the letter's contents.
When the final details are settled, it is understood it will be sent to either The Timesor another national newspaper.
The letter will heap further pressure on Tessa Jowell and Margaret Beckett, whose leaked document proposing legislation to delist the building launched the current controversy.
They argued in the document, which emerged last Friday, that the Grade II*-listed building should be delisted to maximise the profit for the Institute's trustees when they sell the site.
However, critics of this move, including English Heritage and the Twentieth Century Society, were horrified that the government move would completely undermine the listing system, by introducing economic value into it.
News of the letter has pleased these conservationists, who are keen to stress that the Commonwealth Institute case should interest both the architectural profession and the wider community.