The mystery vandal was less than impressed by the revealing artwork at Bath's Victoria Art Gallery and made his own DIY alterations. He glued a black piece of paper on to the portrait to preserve the man's modesty in what gallery bosses later described as a "premeditated attack".

Although the paper has now been removed, the work, entitled 'Male Nude', was damaged.

The renowned artist behind the work, portrait painter David Cobley, whose subjects include Princess Anne, has invited his "critic" to get in touch for a discussion on art.

The 12" by 12" oil painting was hanging in Cobley's exhibition when it was attacked shortly before the exhibition closed on Thursday afternoon.

The man responsible was caught on the gallery's CCTV but has not been traced or reported to the police.

Victoria Art Gallery manager Jon Bennington said:

"The painting has been damaged as the glue used to adhere the paper to the canvas will leave a tacky residue. "It is believed the attack took place just before closing time on Thursday and it was only discovered on opening the gallery on Friday morning."

The artist said the vandal should have discussed his objections with someone at the gallery and described his actions as childish. But he stressed he would still like to talk to him about his portrait.

"I'd like to talk to them about why they find it so bad they have to do this, " he said.