The band has cancelled a number of dates since the end of August (15) after Lemmy suffered breathing difficulties during two shows, which were both cut short.
They blamed his ill health on altitude sickness, and it has now emerged that he was diagnosed with a lung infection.
In a post on Facebook.com, a representative for the band has assured fans the infection has been treated and Lemmy is feeling ready to get back on stage at their concert in St. Louis, Missouri on Tuesday.
The message reads, "St. Louis here we come! The wait is over and the Doc has spoken. Lemmy had a lung infection (exacerbated by that Denver altitude) which has now been taken care of. Subsequently, Lemmy's ready to give it a go in St. Louis at The Pageant theatre on Tuesday, Sept 9th where Motorhead will continue the 40th Anniversary tour...
"Amidst all the wonderful support Lemmy has received, many have rightfully said he owes us nothing as he has given us everything. We, however, owe Lemmy something... our total respect for his fierce, single-minded and beautifully stubborn determination to continue burning asphalt, playing rock'n'roll and live where he has always been most comfortable - on the road!... Thanks again for all your support and see you soon!"
The 69 year old has suffered a number of health woes in recent years, and had a heart operation in 2013.