Speaking candidly about her drug habit for the first time, the actress explained that she first started using the Class A substance when she was in her 20s, but developed a more serious problem in 1989 while suffering from depression following the birth of her first son.
Welch confessed that her "lowest moment" came when she took cocaine in a backstage bathroom at Corrie while her co-stars were busy filming scenes to mark Betty Driver's (Betty Williams) on-screen and real-life 80th birthday celebrations. Her dealer had met her outside the show's Manchester studios moments earlier.
The actress, who played Weatherfield barmaid Natalie Barnes, is quoted by The Mirror as saying: "I'd become the master of getting out of scenes in the Rovers. I'd always be saying something like 'Don't you think Natalie would be in the toilet? Or in the Rovers living room?'
"It was ridiculous, but this was a particularly awful day and I felt very low. When you suffer with depression it can just hit you like that at any time and I could feel myself slipping. I didn't have any drugs on me and I knew the only way I could possibly keep going and stay on an even keel was to get some more."
Welch also recalled arriving at work in July 1998 and barely being able to speak following an overnight cocaine binge. However, she was able to go ahead with her scheduled scene with screen lover Michael Le Vell (Kevin Webster) as planned.
The star eventually kicked her drug habit when she became pregnant with her second son Louis, now 9. She has since discovered that her depression was caused by a hormone imbalance and now suffers low moments less frequently after being prescribed tablets.
Welch reveals the full extent of her personal problems in new autobiography Pulling Myself Together, which is released later this week.