One survey at Kew gardens in west London has found that the number of lichens growing there has more than doubled since 1970, when scientists recorded just 30 species.
The study has so far identified 72 species and is expected to find far more when it is finished this summer. Researchers identified one endangered species which has been seen only 13 times in the past 40 years.
The lichen, Cyphelium notarisii, was found growing on a bench at the botannical gardens, which has now been bolted to the ground to protect its valuable stowaway from being unwittingly removed and cleaned off.
Sandra Bell, a botanist at Kew, said the return of rare lichens to many urban sites shows that certain forms of air pollution are less of a problem than they once were. "Lichens are particularly sensitive to air pollution and so are valuable indicators of air quality."
Lichens are not a single organism but the result of a unique relationship between different types of algae and fungi. The alga, which is a plant, converts sunlight into chemical energy, and the fungus in return provides vital nutrients and physical protection for the alga.
Britain has about 1,700 types of lichen, which can grown on rocks and tree bark. Industrial pollution, however, has severely curtailed their range. In 1820, a lichen survey at Kew recorded 165 species. The number recorded in 1906 was just 15.
William Purvis, head of lichenology at the Natural History Museum in London, said that lichens are particularly susceptible to acid rain, caused by the release of sulphur dioxide by open fires and coal-fired power stations.
Since the Clean Air Act of 1956 - intended to combat London smogs - and the closure of many coal-fired power stations, levels of sulphur dioxide released in Britain have fallen significantly, allowing lichens to re- colonise old haunts. Dr Purvis said: "UK emissions of sulphur dioxide have decreased by 80 per cent since 1962. As a result there has been a remarkable increase in lichens. Every year we find new species."
The demise of lichen-covered trees in Victorian Britain was responsible for a famous cases of evolution in action - the demise of the mottled form of the peppered moth in favour of its black, melanic mutant.
Scientists found that the mottled form was perfectly camouflaged against predators when resting on a lichen-covered tree. When lichens disappeared, it was the relatively rare melanic form that had a better chance of not being seen by birds.