Stamp, one of Britain's leading architectural historians, says: "It is a late work by a distinguished architect, W Curtis Green, and is now a rare surviving example of an interesting type: commercial buildings erected after 1945 designed by good, so-called traditionalists who remained true to their principles while adapting to modern conditions. It ought to have been listed; it ought not to be demolished."

The lobby group Save Britain's Heritage still hopes to fight Legal & General's demolition plans. Adam Wilkinson, the group's secretary, said: "It should be listed. This is a great building and we just can't understand it."

A spokeswoman for English Heritage says it hopes to leave its headquarters in the next 12 months, but denies a conflict of interest in the decision.