Image caption Hefner admits affairs with many Playmates over the years but vowed fidelity to Harris in April

Hugh Hefner, the 84-year-old founder of Playboy magazine, has announced that he is to be married for the third time.

In a message to his Twitter followers on Christmas Day, Hefner said he had given his 24-year-old girlfriend, model Crystal Harris, a ring.

"Yes, the ring I gave Crystal is an engagement ring," he clarified in a later tweet.

Hefner set up Playboy magazine in 1953, and Harris featured in it as Playmate of the Month in December 2009.

The soft-porn magnate and libertarian has been married twice before - to Mildred Williams in 1949, and Kimberley Conrad in 1989.

Over the decades he has admitted affairs with dozens of Playmates.

But last April Hefner and Harris reportedly "became exclusive".

"When I gave Crystal the ring, she burst into tears," Hefner told Twitter followers.

"This is the happiest Christmas weekend in memory."

For her part, Harris told Twitter followers simply: "The most memorable Christmas ever."