A vandal managed to carve 'Tony Blair is a cunt' into the Commons table used by the Prime Minister.
The culprit - thought to have been on a sightseeing tour - etched the words into the wood unnoticed.
Conservative MP Derek Conway says furniture polishers were brought in to remove the message, but Wednesday's incident has led to a change in security.
Mr Conway, chairman of the Commons Works Committee, says hard plastic covers will now be put on the despatch boxes on both sides of the House when visitors are around.
He said: "Whoever this bright clown was probably thought he had struck a blow for freedom when all he has achieved is to hamper the hundreds of thousands of people who want to walk through the Chamber."
He said that in future they would not be able to linger so long in that area, adding that he did not envisage any other restrictions for visitors. "The culprit appears to have been a visitor rather than a pass-holder.