Lindsay Lohan’s “relapse” may have been sparked by on-off boyfriend Calum Best after the British reality TV star reportedly dumped her for being “too boring” while sober.

Lindsay and infamous party animal Calum are said to have enjoyed a brief fling last year when Lindsay was in London for the World Music Awards. They reunited in New York in May, and were rumoured to have just put their relationship on hold while Lindsay was in rehab. Calum attended Lindsay’s (supposedly sober) 21st birthday party earlier this month and days later, Lindsay admitted to having a new boyfriend.

However, now the Daily Star has claimed that Calum - who had pledged to go into rehab himself for cocaine and sex addictions - found post-rehab, alcohol-monitored Lindsay to be dull.

A source told the newspaper: “Calum called Lindsay on Friday and said that he was finding her post-rehab existence dull and that he didn’t want to give up partying.”

“Calum loves Lindsay, but he’s simply not in the same place as her anymore.”

Lindsay’s lawyer Blair Berk said yesterday that he had been getting “timely and accurate reports from the testing companies” monitoring Lindsay’s sobriety through her SCRAM anklet but that he found out on Monday afternoon that she had “relapsed” over the weekend.