Mr Birchenall, known to supporters as ‘The Birch’, was about to present the newspaper’s Club of the Year honour when he complained of feeling unwell.
He then collapsed at the back of the presentation stage at rugby club Leicester Tigers’ Welford Road stadium and was treated by off-duty hospital staff, who were at the event, before paramedics arrived.
He was then taken to Leicester’s Glenfield Hospital to be treated by heart specialists, and the rest of the event was abandon.
Mr Birchenall played for clubs including Chelsea and Leicester during his professional football career, and now works for the latter club in an ambassadorial role which includes serving as a pitchside announcer on matchdays.
He also completes an annual charity run before the last home game of each season.
Mercury editor George Oliver told HTFP: “There has been a massive outpouring of love and support for Alan since he was taken ill last night.
“The Birch has long been a key part of the Mercury’s sports awards and was last year presented with the lifetime achievement award to mark his long association with Leicester City and his work for charity.
“Everyone at the Mercury is thinking about him and wishing him a speedy recovery.”
Rangers footballer Martyn Waghorn, who used to play for Leicester, posted on Twitter: “Come on Birch, stay strong mate. Was the first face I saw heading into [the club] and welcomed my family in. Legend.”
The decision to abandon the ceremony was taken in agreement with sponsors, the club and the organisers of the event, and a follow-up is being arranged.