NEW YORK (AP) _ Heavyweight champion Mike Tyson, undefeated in 35 professional fights and coming off the biggest payday of his career, says he will not fight again.

''You can't never say never,'' Tyson told the New York Post's Mike Marley. ''But as of today, I am retired.''

Tyson, relaxing at his home in Bernardsville, N.J., after his first-round demolition of Michael Spinks, said:

''I had fun. Boxing was good to me, but nothing lasts forever. After this fight, I thought it over. I decided it's time to move on. It's time to call it a day.''

Tyson conceded that skeptics might not believe him, but he was insistent on his plan.

''They don't have to buy it,'' said Tyson, who scored his 31st knockout against the previously unbeaten Spinks Monday night at Atlantic City, N.J. - a fight for which the champion is expected to receive more than $20 million.

''They're not getting in the ring and doing the fighting I am. I'm willing to sacrifice, to give up boxing to save my family. ... I just feel it's the best thing to do.

Tyson said he ''heard (promoter) Don King and his guys laughing'' when he made the comment about retiring following the Spinks fight.

''When he finds out I'm serious, he won't be laughing,'' Tyson said.

King was quoted in today's New York Times as saying, ''I think Mike Tyson needs a rest, which is what he calls retirement. He's fought for three years nonstop.''

Tyson had indicated after the match against Spinks that it might be his last fight, but he was not as emphatic as his comments which appeared in today's edition of the Post.

''As far as I know, this may be my last fight,'' he said.

However, he is tentatively set to fight England's Frank Bruno on Sept. 3 in Londojn.

Tyson confidant Jose Torres said he was confident the Bruno fight would be held.

''He's going to fight Sept. 3,'' Torres said. ''I'm positive.''