Haskell, who won 77 caps for England and played in two World Cups, announced his retirement from rugby in May at the age of 34.
He has been training alongside current MMA star Michael Page in London, and will compete in the heavyweight division.
Haskell, who also enjoyed Premiership and Heineken Cup success with Wasps, is expected to reveal more about his switch at a press conference in London next week.
Other stars to switch sports
Here are some other stars who have switched sports - with varying degrees of success.
Usain Bolt - athletics to football
Bolt's professional football aspirations lasted all of eight weeks. The world's fastest man signed a two-year contract with Australia's Central Coast Mariners but amid criticism over his ability, he quit the deal early and announced his retirement.
Sonny Bill Williams - rugby to boxing
Not content with switching between rugby league and union codes, the All Blacks Centre also embarked on a career in professional boxing. He was unbeaten in seven fights between 2009 and 2015 and also held the New Zealand heavyweight title.
Michael Jordan - basketball to baseball
Having won his third successive NBA title with Chicago Bulls in 1993, Jordan embarked on a minor league baseball career. He played for the Birmingham Barons and the Scottsdale Scorpions but after two largely unsuccessful years, he returned to the NBA.
Victoria Pendleton - cycling to horse racing
Double Olympic gold-winning cyclist Pendleton swapped saddles to become an amateur jockey. She won her first race at Wincanton in 2016 and went on to place fifth in a race at the Cheltenham Festival, calling it "probably the greatest achievement of my life".
Dame Sarah Storey - swimming to cycling
After winning six swimming medals at the Barcelona Paralympics at the age of 14, persistent ear infections prompted Storey to switch sports to cycling. She went on to become the most successful female British Paralympic athlete of all time - and was made a Dame in 2013.
Rebecca Romero - rowing to cycling
As a rower, Romero won silver at the 2004 Olympics and gold at the following year's World Championships. When a persistent back injury forced her to stop
competing, she swapped boat for bike. In 2008 she became the first British woman to win a medal in two different Olympic sports - this one was gold.
Paolo Maldini - football to tennis
Defender Maldini won almost everything at AC Milan, but his professional tennis career was another matter. After a 6-1 6-1 defeat in the doubles tournament at an ATP Challenger Tournament in Milan, he admitted: "Those who know sport know very well that it's impossible to invent yourself as a professional from one day to the next."