Toad numbers have fallen by more than two-thirds in 30 years, according to a study using data from volunteer patrols set up to help the amphibians cross roads.
While the reckless but lovable Mr Toad in Wind In The Willows is a fan of motor cars, in real life common toads are vulnerable when crossing busy roads as they migrate to their breeding ponds.
Volunteers for Froglife's "Toads on Roads" scheme carry tens of thousands of toads across roads around the UK each year.
Now the charity's scientists have teamed up with experts in Switzerland to assess information on the number of toads transported across roads by patrols in the two countries, to chart the fortunes of the species.
In the UK, data collected from 153 sites stretching back to 1985 has revealed a 68% decline in the past three decades.
There were declines in almost all regions in all decades, with conservationists warning it was likely hundreds of thousands of toads had disappeared from the countryside in the past 30 years.
South East England saw the worst declines, while in Wales, south west and west England populations had also declined but have been stable for the past decade, the study published in the journal Public Library of Science ONE found.
In the Eastern region severe declines in previous decades were followed by a recovery since 2005, but not enough to reverse overall falling numbers, while central and northern areas and Scotland have also experienced declines.
The reasons for the falling numbers are not clear, but likely causes are thought to be changes due to farming practices, a loss of ponds, urban sprawl and more deaths on roads as traffic increased.
Climate change could also be a factor, the experts warn, as research has shown that milder winters have a negative impact on hibernating toads.
Dr Silviu Petrovan, conservation co-ordinator at Froglife and one of the authors of the study, said: "Toad declines at this scale over such large areas are really worrying.
"Toads are extremely adaptable and can live in many places ranging from farmland and woodland to suburban gardens.
"They are also important pest controllers eating slugs, snails and insects and are food themselves for many of our most likeable mammals such as otters and polecats.
"Without the efforts of the thousands of volunteers that go out and move amphibians across busy roads we would have no idea that these declines had occurred and the situation could be much worse.
"One thing that is clear is that we need to do more to look after our environment in order to protect the species that depend on it."
Paul Edgar, senior amphibian and reptile specialist from Government conservation agency Natural England, said: "The common toad is sadly on a downward trend.
"This is partly because of habitat fragmentation, and so understanding and mitigating the impacts of this issue is vital.
"We need to continue to build good quality habitat links across the wider landscape if we are to offer opportunities for this species to recover."