LONDON (Reuters) - The pseudonymous British graffiti artist Banksy has withdrawn an offer of free artwork in return for tactical voting in Thursday’s general election after police warned it would invalidate the result.

The artist, who found global fame with his pop-up street art, offered prints of his famous “girl with balloon” for those who voted against Prime Minister Theresa May’s Conservative party in six constituencies near his Bristol home.

Voters were supposed to provide photographic evidence of their ballot to get the print, but the secretive Banksy withdrew the offer on Tuesday after police said it was a criminal offense to accept gifts in return for votes.

“I regret to announce this ill-conceived and legally dubious promotion has now been canceled,” the artist, whose work often features a political edge, said in a statement.

Five of the six constituencies where Banksy’s art offer had applied were held by the Conservative party, who have a narrow lead over Labour in polling before Thursday’s election.