British actor Daniel Radcliffe has stepped up his security after Harry Potter film studio Warner Bros received a death threat directed at the young star from a stalker. The 18-year-old is alleged to have temporarily replaced his regular driver after Warner Bros assigned four ex-army men from the U.K.'s Special Air Service Regiment (SAS) to follow his every move, escorting him to and from the set of the latest boy wizard movie Harry Potter And The Deathly Hallows. According to British newspaper The Sun, the nature of the threat is unclear, but it is being taken very seriously by studio executives.. A source says, "There is a real concern for Daniel's welfare. The bigwigs haven't said what has been going on. It's all hush-hush - but security has been massively ramped up... "Daniel has the four guards with him at all times, as well as a new driver. When he travels they use dummy cars and swap vehicles." A representative for Warner Bros has refused to comment.