The 47-year-old former England captain said it had been “incredibly hard to process and a huge shock to me and my family”.
In a social media post, he wrote: “I’m writing to share some tough news. I have recently been diagnosed with ALS, also known as Motor Neurone Disease (MND).”
He said: “I feel fit and well in myself and I’m focused on staying positive, living life and dealing with the changes I will experience as they come.
“I am being well supported by my family, friends and medical professionals and I’m truly grateful to those who, in their time, helped progress research to support others, like me, living with this disease.
Since retiring from the sport in 2012, Moody and his wife, Annie, have dedicated much of their time to fundraising for The Lewis Moody Foundation, which supports those affected by brain tumours.
“My plan is to continue with this, but to also create an opportunity to support a charity closer to my current situation,” said Moody, who was awarded an MBE for services to rugby in 2004.
“I would be so grateful for your help with this and look forward to sharing more, once I am clear on what this looks like.”
Rugby players may be more prone to motor neurone disease
Fellow rugby players Doddie Weir and Rob Burrow have died from the disease in recent years.
The sport – and in particular England skills coach Kevin Sinfield – have worked on high-profile fundraising campaigns to tackle it.
Athletes appear to be disproportionately likely to contract MND, which causes muscle weakness to the point it can eventually be hard to eat or breathe.
Research from Durham University found rugby players may be especially prone, as those who have suffered multiple concussions have higher levels of certain proteins in their blood that are linked to the disease.
But the exact cause of it remains unknown.
Read more: ‘Game changer’ test could spot motor neurone disease
“For now, please know I feel your love and support,” Moody added.
“All I ask is that I am given some space to navigate this with my wife and sons, and those closest to us – but without doubt, I will continue to embrace life and grasp opportunities in the same way I always have.”
A ‘glittering career’
Moody won 71 caps for England and also five for the British and Irish Lions, and was a seven-time title winner with Leicester.
He is perhaps best known for winning the final lineout during the 2003 World Cup final, which led to Jonny Wilkinson’s historic match-winning drop goal.
Lions Rugby Chair Ieuan Evans MBE said: “We are all greatly saddened to hear the news of Lewis Moody’s diagnosis.”
He added: “As a player, Lewis inspired so many fans during a glittering career.”
Moody joined Leicester Tigers from Oakham School and made 223 appearances between 1996 and 2010.
The club said everyone there was “deeply saddened to learn that one of our greatest players” had been diagnosed with MND.
“The figures, trophies and awards tell you what an incredible player Lewis was, but that is only half the story,” said Tigers chief executive Andrea Pinchen.
“As an individual, his commitment to his club along with his warmth and passion shone through, which endeared him to teammates, staff and supporters alike.”
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