The report concludes that no individual was to blame for the blaze, which wrecked six rooms and caused damage unofficially estimated at pounds 60m.
Ann Clwyd, Labour's National Heritage spokeswoman, described the report as a whitewash and called for a full inquiry. The Liberal Democrat's National Heritage spokesman, Robert MacLennan said: 'To suggest this is the only report we are going to get is completely unacceptable.'
A five-strong team of police, forensic experts and fire officers spent eight days investigating the fire and carried out more than 100 interviews. Their report says the fire started on the first floor of the Private Chapel close to the altar. The chapel was being used as a temporary store and restoration area for paintings at the castle.
The report, released by Buckingham Palace, said seven possible causes were considered before it was concluded that the most likely was a high-level spotlight setting fire to curtains next to the altar.
It says: 'Eye-witness accounts confirmed this as being the approximate location in which flames were first sighted.
'The team were also able to confirm that the spotlight had recently been refitted - but that its angle and direction of facing could not be established - and that the curtaining had probably been pushed towards the lighting by a large picture frame having been placed up against it.'
Buckingham Palace said the one-and-a-half page report, delivered yesterday, had been seen by the Queen. 'The fire appears to have resulted from an unfortunate combination of circumstances with no single factor or individual directly to blame,' it said. 'Further investigation into the cause is not thought necessary.'
There was satisfaction in royal circles that the report absolved Viola Pemberton-Pigott, the chief picture restorer, who was accused in several newspapers of being responsible for the fire.
Ms Clwyd was far from satisfied. 'I can't believe this is the report. This seems more like a heavily edited extract. We heard eye witness reports that fire extinguishers didn't work. We need to know if they did. None of the real questions have been answered.'
Following the fire at Hampton Court, an inquiry recommended regular fire drills at Royal residences. On 23 November Ms Clwyd asked Peter Brooke, Secretary of State for the National Heritage, to detail the number of those undertaken at Windsor Castle. She has not yet received a response.
The Fire Protection Association said sprinklers would have confined the fire to one wing. Lessons from fires at Hampton Court and York Minster were not learned, it said. 'You cannot eliminate every single fire risk, it would be impractical. The mistake was failing to install sprinklers.'
The day after the fire, Mr Brooke, said sprinklers were impractical at Windsor because water would damage priceless artefacts, but fail to tackle blazes behind panelling.
However, the FPA said: 'If something gets wet, you can repair it. If it is burned, nothing will bring it back.'
Clearance work at the castle is still going on and a temporary roof is being erected this week.