A bus passenger could scarcely believe his eyes when he saw mushrooms growing on its upper deck.
Kevin Bullock, from Chesterton, was on Stagecoach's number 15 service on Friday morning and noted some crisps and papers on the floor upstairs when he took a seat.
But when he looked more closely at the area beneath the windscreen, he took pictures and sent them to the News.
He said: "I was disgusted to see that growing on the bus near the front main window were mushrooms. Rubbish I can understand on a bus but mushrooms growing is unbelievable.
"There are obvious health and safety concerns for parents with young children who want to sit at the front on the top deck."
Andy Campbell, Stagecoach Cambridgeshire's managing director, said: "I'm amazed. I've never seen anything like it.
"There had been a repair to the upper saloon of the bus, it was not of the quality expected and there was water ingress.
"I'm not an expert on mushrooms but I understand these things can grow overnight. We will have the mushroom tested and we've had the vehicle taken off the road."
Mushroom expert Dr Harry Hudson, a retired Cambridge University academic, studied the pictures and told the News:
"The fungus in question is Coprinus cinereus, one of the Agaricales or a toadstool commonly called "ink caps".
"The fungus is common on compost heaps and rotting straw as its main source of nutrition is cellulose and lignin. It grows well on chipboard and plywood, if it becomes moist or wet.
"I did some research in Cambridge in collaboration with a colleague in the University of Aston on this fungus. We produced some edible fungal protein, not unlike Quorn, by growing this fungus on straw with urea as a nitrogen source.
"I would not suggest anyone should eat the fungus - autolytic products could cause a mild stomach upset, but it is certainly not poisonous."
He said the best known member of the genus is Coprinus comatus, also called Lawyers' Wigs, common on decaying cut grass, roadside verges and roundabouts in the early autumn.
He said: "This can be described as 'edible and good' when young."