London: Officials are used to finding graffiti on the priceless fixtures and fittings of the House of Commons chamber.

Teams of French polishers regularly examine damage done by schoolchildren who visit when the parliament is not sitting. But during a recent examination they noticed that the government Dispatch Box was covered in strange black pen marks. To the horrified officials, it seemed that only one member of the government could have been responsible: Gordon Brown.

And so, at the next Prime Minister's Questions session, badge messengers in tail coats were posted in the gallery above to spy on the member in question as he rose to speak.

As he gesticulated wildly with his black marker pen, stabbing at the papers in front of him and missing, hitting the wood beneath, the awful truth was clear. Gordon Brown had been vandalising the Dispatch Box.

For how long, officials are still not clear. But the French polishers were unable to remove many of the deep markings.

An official explained: "If he goes on doing it, it is going to be harder and harder to get off."

The dispatch boxes, a gift from the government of New Zealand after the World War II, are modelled on the dispatch boxes in the Australian parliament. They are made from a strong, teak-like wood from the rare puriri tree and were thought to be almost indestructible.

Black marker

The Prime Minister scribbles furiously while his opponents are speaking and his papers often show a mass of black pen markings where he has crossed out and scrawled all over his prepared speech. He uses a black marker pen because he is blind in one eye.

An MP close to Brown said: "Gordon is known as Zorro for getting his sweeping black pen everywhere, but I'm sure he'll take greater care around the nation's artefacts now he has been warned."