The number of wild farmland bird species breeding in England is at the lowest level since records began, a key government wildlife "indicator" shows.
Birds, like the skylark, are seen as an indication of healthy habitats
The RSPB called the UK government's Wild Bird Populations 2006 indicator "extremely depressing".
The data showed that these species had declined by about 60% since 1970.
The charity warned that cuts in "set-aside" payments, which take land out of food production, could hit bird numbers even harder in the future.
Sue Armstrong-Brown, the RSPB's head of countryside policy, said: "Farmland birds are the barometer by which the government measures the health of the countryside.
"We wish there was a better story to tell, but the farmland bird index reaching its lowest point is extremely depressing news," she added.
"Specialist" species, such as the skylark, which are dependent on farmland habitats for breeding and feeding, have suffered the greatest decline.
The farmland bird population index, which measures 19 species, showed a decline of more than 50% in bird numbers between 1977 and 1999, but then stabilised at this level.
The yellowhammer is one of the birds that has seen its numbers fall
"However, the indicator has fallen slightly in recent years," the document published by the Department for Environment, Food and Rural Affairs (Defra) stated.
"[But] it is too early to say whether this is the start of a further period of decline."
Defra suggested that the decline of species included in the index was a result of changes to agricultural processes, "including the loss of mixed farming, the switch to autumn sowing of cereals... and the loss of field margins and hedges".
Since the 1990s, farmers have received payments under the Common Agricultural Policy (Cap) to set aside some of their land and take it out of food production.
Dr Armstrong-Brown feared that EU plans to phase out these payments would have a "devastating effect", despite the introduction of other agri-environmental plans.
"The decision to reduce set-aside to zero this year throws a spotlight on the importance of individual farmer decisions in caring for birds on their land," she said.
"We want to work with the farming community to make sure that the trend from here is upwards."
Gareth Morgan, head of the RSPB's agricultural policy team, said the UK had developed some of world's best examples of agri-environment schemes.
"More resources for these schemes are urgently required if we are to stand any chance of halting and eventually reversing the decline of some of our best-loved birds," he warned.
Other bird populations covered by the indicator are: woodland birds, which have seen a 20% decline since 1970; and seabird species, which have risen by 30% over the same period.
The indicator forms part of the UK government's sustainable development strategy, as bird populations are considered to give a good indication of the health of the nation's wildlife and habitats.