The blaze destroyed the old Commons debating chamber before firemen put it out after two hours.
Unionist suspicions that the fire may have been started deliberately were rejected by Malcolm Moss, the Northern Ireland Office Minister, who said there was no evidence to suggest arson.
James Kilfedder, a former Speaker at Stormont and chairman of the Westminster Select Committee on Northern Ireland Affairs, demanded a full investigation.
The blaze was spotted by security men on duty inside the buildings and two Royal Ulster Constabulary officers exercising police dogs in the vast grounds outside.
The building has never had a sprinkler system, but one was due to be fitted as part of a £3m refurbishment to be completed next summer.