The pop star began dating Smith, a former school classmate from his English hometown, in August.Payne sparked rumours that they had split when he wrote a series of sad posts on Twitter on Friday, which read: "Can't be without you... Worse thing I ever did was let you go... No (sic) all I got left is the thought of you."Reports suggest the boyband's world tour, which began in South America last month, is at the root of the problem.A source tells The Sun: "Liam said she is the sweetest girl he's ever met and he's really sad about the split. "But he knows it was the right thing to do. He wants her to enjoy university and not be sad waiting for him to come back from tour for the odd day here and there."