Plants in Britain are flowering earlier today than at any period since records began, according to data collected by ordinary gardeners over 250 years.

In an example of “citizen science”, researchers looked at more than 400,000 records collected by Victorian vicars, dog walkers and the viewers of the BBC series Springwatch.

The records, that go back to 1753, were divided into 25 year periods in order to compare the average flowering time as the weather changed from year to year. More than 400 species were recorded, from spring blooms like daffodils to summer plants like ivy.

In recent times, from 1985 to 2009, flowers came up on the 20th May, earlier than during any other period.

In the coldest 25-year period, from 1835 to 1859, plants bloomed almost two weeks later around 3rd June. In even the warmest period compared to today, from 1910 to 1934, plants came up two days later on 22nd May.

The figures, analysed by scientists from around the world including Cambridge University, found that on average plants flower five days earlier for every 1 degree C (1.8 degree F) rise in temperature.

The result, published in the Proceedings of the Royal Society B, is the first time that scientists have managed to calculate how many days earlier the average plant will flower as the temperature increases.

The study of how plants react to the climate, known as “phenology”, is important because of the knock-on effect for other species. For example pollinators such as bees and butterflies may struggle if flowers bloom early and are over quickly, threatening animals further up the food chain and even humans.

Researchers hope the results will help to work out a “global index” showing how many days earlier plants will flower around the world as the temperature warms, so it is possible to protect certain species.

Richard Smithers, Woodland Trust Senior Conservation Adviser and one of the authors of the paper, said the study showed how climate change is already affecting Britain.

“It is hard to make climate change real for people - they see a winter like this and do not believe it. This makes it very real for people. The natural world is our canary in the cage. It does show what is happening and the fact that plants flower five days earlier for every 1C of temperature rise and the fact that the last 25 years was earlier than any other period since 1753 should tell people a great deal,” he said.