The DIY SOS star — who is on hiatus from the BBC show after he breached advertising guidelines by starring in and ad for Nestle breakfast cereal Shreddies — has confessed he caught coronavirus back in 2020 which left him feeling weak and fatigued for 10 months.
Knowles, 59, told ITV’s Lorraine: "Yeah, I had [conovarius] really bad. Right at the start of lockdown, I got COVID pretty bad.
"And me being impatient, I tried to get back into exercise straight away afterwards.
"There seems to be some sort of correlation, and now it seems to be said that going too hard, actually exercising too hard, too soon after COVID, can actually cause you problems.
"And I had long COVID for about 10 months — no energy at all, just knackered, but I did not want to tell anybody because I thought I may not get a booking for work. So I kept it to myself."
The former I'm A Celebrity... Get Me Out Of Here! contestant confessed he would struggle to move around building sites while filming, and so he would tell his colleagues he was stopping to "take in the view", when really he needed to catch his breath.
He said: "I'm climbing up huge buildings, seven storey buildings and having to stop every two ladders."
A joint statement from the BBC and Knowles said: “Both Nick and the BBC have resolved the issue recently reported relating to Nick’s commercial agreements and BBC guidelines.
“With the campaign still running, Nick will step back from the DIY SOS Children In Need special this year but will be back on our screens in 2022.”
Knowles has been the main presenter of the BBC One home renovation show since 1999, in which a team of builders help improve a family's life by giving their home a full construction makeover.
The last series of the show aired on the BBC between November 2020 and February this year.
Watch: Nick Knowles broke BBC guidelines by appearing in a Shreddies ad