Had my daughter not phoned as I was getting ready for work, I'd never have been on the 145 to Dagenham, London at just after six that evening – usually, I take a later bus. But the route to Dagenham Trades Hall, where I'm a barmaid, goes past where she lives, and she wanted me to pick something up for my grandchildren's dinner.

It's an unremarkable journey I've taken countless times before, always sitting on the lower deck. Things started to take an unusual turn only when the young woman sitting in front of me asked if she could borrow my phone. I'd noticed her as I got on – late 20s, dark hair, heavily pregnant. Very heavily pregnant, it turned out. "My waters have broken," she said. "I need to phone my friend."

"I think we should call an ambulance," I said. But she was insistent. "I've got no clothes with me, no books. I need to let someone know." Clearly in some discomfort, she made the call with admirable calm. But as she handed it back to me, she gasped. "Oh!" she said. "I've got more water coming out…"

My husband is a bus driver and sometimes works the same route, so I knew the driver, and the jargon. I walked to the front of the bus and tapped on the glass. "Keith," I said, "we've got a code red." He pulled in at the next stop and called for an ambulance. Meanwhile, the woman found herself a spot on the floor by the back doors. I asked her about herself and found out she was called Joanne, that this was her first baby and that she was a week overdue. She'd been heading home from shopping in Ilford.

"I need to start pushing," said Joanne, who appeared far more composed than I was feeling. I told her to hang on for the ambulance. "I can't, the head's coming out!" she said. I have had two children, and it all seemed a bit quick to me. I felt down between her legs and, even through her jeans, it was clear she was right. I took a deep breath. "Right," I said, "we need to get these trousers off." Despite the driver's best efforts to evacuate the bus, there were still passengers hanging around, and she was too embarrassed.

I may not have any experience as a midwife, but I know how to clear a bar. "Will you all move away and let us get on with delivering this baby?" I shouted. I pulled down Joanne's jeans and the baby slid into my arms and immediately started crying. It was a boy, quite small, with surprisingly thick, black hair. I couldn't believe how quickly it had happened. I had two long hospital labours, and though I wouldn't have wanted to give birth in public, I'd have been delighted to have encountered such ease. No more than 15 minutes had passed from the time Joanne had asked for my phone, to the point when I held her baby.

Another passenger gave me her cardigan to wrap the baby in. As I handed him to his mother, Joanne started shouting, "What are you doing?" I looked behind and saw a man using his phone to take photos. I gave him a few choice words – he didn't hang around. As we waited for the ambulance, the baby became quiet, and started to turn blue-grey. Instinctively, I reached over and rubbed his head and cheeks. "Why is my baby so quiet?" Joanne asked, perhaps sensing my rising panic. I told her I was trying to get a response, and carried on massaging. All at once, his colour returned and he started crying again. Apparently newborns can sometimes have problems with irregular breathing.

The ambulance crew arrived and made straight for Joanne, who sat peacefully holding her baby. Another 145 pulled in and, not wanting to be late for work, I got on it, calling a friend en route to ask them to deliver my grandchildren's supper. A couple of women who had been standing nearby during the birth sat by me, and told me they couldn't have done what I had. Only then I realised I was shaking uncontrollably.

"You'll never guess what I've just done," I said to the other barmaid when I arrived. She sat me down and gave me a drink. When I got home that night, I burst into tears, thinking about all the things that might have gone wrong. I later found out this was only the second baby ever born on a London bus.

Joanne has since been in touch to thank me – she's called her little boy Joaquin, and he's 7lb 14oz. She says she'll bring him round to see me.

• As told to Chris Broughton

Do you have an experience to share? Email experience@theguardian.com