Charlie Sheen has apologised for saying Ashton Kutcher "sucks".

The actor recently made derogatory comments about his successor on the US TV show Two and a Half Men, saying both Kutcher's acting and the script writing were bad.

But now the eccentric star has taken back the insulting remarks about Demi Moore's estranged husband in an official letter.

"Dear Ashton, my bad. I was disrespectful to a man doing his best. I got excited and I threw you into the crossfire, but the rest of my statement I stand behind. You however, deserve better. Safety in your travels good sir. CS," he said in the apology posted on TMZ.

His remorseful address is a far cry from his original rant, which was also published on the website following a telephone interview with the actor.

Sheen was fired from his role on the popular programme last year, after he fell out with producer Chuck Lorre and sparked concern with his controversial private life.

Ashton Kutcher replaced Charlie on the programme portraying a different character - and initially Charlie came out applauding his performance.

However, earlier this month he was less than flattering about the star.

"I'm tired of pretending Ashton doesn't suck. I'm tired of lying... I'm tired of pretending the show doesn't suck," he said.

Charlie explained that he had attempted to be politically correct when Ashton joined the show but revealed his true feelings when Warner Bros. Television studios threatened legal action against him for using a Two and a Half Men photo to promote his new series Anger Management.

"It's nothing personal... I just feel bad for [Ashton]," Charlie said. "[H]e's saddled with such bad writing."