The gallery said it did not know why the painting was attacked

The oil painting of Samuel Johnson by Sir Joshua Reynolds was hit several times smashing the glass and leaving the hammer embedded in the canvas.

A spokeswoman for the London gallery said the painting was reparable and is currently being examined by experts.

Police said a man, aged about 35, was arrested at the gallery on suspicion of causing criminal damage on Wednesday.

The suspect was held by security staff before police arrived at 1747 BST. He remains in custody at Charing Cross Police station.

Cost of repair

The painting, said by the gallery to be one of its most popular, is part of the permanent collection on the top floor.

The gallery spokeswoman said they were happy with the security arrangements but added there were no bag checks in place.

She said there was no obvious reason why the Samuel Johnson painting was singled out.

"Restoration experts are examining the painting and until they have completed that it is difficult to know how much it will cost," she said.