Lindsay Lohan’s boyfriend Jared Leto has been spotted ‘all over’ a mystery blonde – sparking rumours he has cheated on the actress.

The heartthrob actor was reportedly seen at the Hard Rock Hotel & Casino, in Las Vegas, cosying up to the sexy girl after watching a performance of popular show ‘Madhouse’, according to America’s New York Post.

Onlookers claimed Leto was flirting outrageously with the blonde beauty and the pair seemed to be getting very close.

News of Leto’s alleged liaison is sure to come as a blow to Lohan who has allegedly been telling friends the actor is ‘the one’.