Kennedy headed the Lib Dems from 1999 to 2006 and is currently MP for Ross, Skye & Lochaber. A poll in his constituency found he could be defeated by the SNP's Ian Blackford, whose support was put at 40% to Mr Kennedy's 35%.
The 55-year-old appeared alongside leader of the Greens Natalie Bennett, Labour MP Lucy Powell, Tory minister Anna Soubry and editor of Private Eye Ian Hislop.
Twitter was divided over the Scot’s appearance on the Leeds-filed debate, with many attacking the BBC:
I don't think the BBC is safeguarding the safety of Charles Kennedy.— Michael Rosen (@MichaelRosenYes) March 12, 2015
People tweeting about Charles Kennedy being back on the bottle. Addiction is an illness, if someone slips they shouldn't be ridiculed #bbcqt— Andrew Neary (@andrewneary) March 13, 2015
As a recovering alcoholic, seeing Charles Kennedy on #bbcqt made me sad. There go I but for the grace of God and all that. Get well soon sir— rob collins (@rwpcollins) March 13, 2015
Charles Kennedy is a thoroughly decent chap.— Joel Taylor (@JoelTaylorMetro) March 13, 2015
Not a fan of Lib Dems but felt for Charles Kennedy tonight. Needs help not mocking.— spicedrummum (@spicedrummum) March 13, 2015
Dismayed by some of the Charles Kennedy comments. The man has previously admitted to a ruinous disease. #bbcqt— Christopher Gage (@ChristophGage) March 13, 2015
Charles Kennedy not at his best on #bbcqt... Hm.— Paul Brand (@PaulBrandITV) March 12, 2015
Seeing a lot of mocking towards Charles Kennedy.
A good man who needs a little help. Like many people do now and again.— Stan Collymore (@StanCollymore) March 12, 2015
I’m not sure letting Charles Kennedy on #BBCQT was in anybody’s best interests, to be honest.— Dave Jones (@WelshGasDoc) March 12, 2015
Charles Kennedy on #bbcqt. It’s making me sad.— SimonNRicketts (@SimonNRicketts) March 12, 2015
I think we're all waiting for that sudden bang when Charles Kennedy falls off his seat and the panel all pretend not to notice. #BBCQT— Matt Malone (@MattMaloneMD) March 12, 2015
I cannot believe the BBC would allow a completely drunk Charles Kennedy to go on. How sad. #BBBQT— Alex Andreou (@sturdyAlex) March 12, 2015
Charles Kennedy... I'm not going to say it, but... #bbcqt— Toby Young (@toadmeister) March 12, 2015
If Charles Kennedy was wearing sad-faced clown make-up, he couldn't look more miserable. #bbcqt— Matthew Sweet (@DrMatthewSweet) March 12, 2015