Droughts could devastate food production in the England by the 2020s, according to a report from the government's official climate change advisers. Without action, increasingly hot and dry summers may mean farmers will face shortfalls of 50% of the water they currently use to grow crops. The report, from the climate change committee (CCC), also warns that current farming practices may be allowing the country's richest soils to be washed or blown away.

The future risks to England's food supply are becoming more apparent, with MPs warningthis month that the government's failure to protect the most valuable farmland from flooding "poses a long-term risk to the security of UK food production" and food experts cautioning that crop yields are reaching their maximum biological limits. Extreme recent weather – the wettest recorded autumn followed by the coldest spring in half a century – cut wheat yields by one third, leading to the import of 2.5m tonnes of wheat, the same amount that is usually exported.

"If we don't start acting now we will be in serious trouble," said Lord John Krebs, who led the CCC report. "We already rely on food imports to a significant extent." About 40% of the UK's food is imported.

Despite recent gloomy summers and suggestions of more to come, most climate scientists are confident that summers in the medium term will become drier and hotter. The CCC found that a dry year in the 2020s could see an irrigation shortfall of 120bn litres, half the current total used by farmers. Furthermore, those areas most at risk of drought – the fields of east and southern England – are currently the most productive.

To avoid this, farmers will need to build twice as many reservoirs on their land than exist now and also cut by 50% the amount of water used per hectare. Ensuring that the cost of water reflects how scarce it is, is also crucial, according to the report. At present, for example, the cheapest water by far is provided by Anglian water, despite that region being one of the driest in the country.

But if the threat of drought can be avoided, said Krebs, climate change could represent an opportunity for farmers in England because there will be longer, warmer growing seasons. The Department for Environment, Food and Rural Affairs (Defra) published its strategy to adapt to global warming on 1 July and, while Krebs said it covered all the key areas, he added that it contained very few targets. "If it is just 'talks about talks', that is not going to help the country adapt," he said. Krebs said new water abstraction rules, unchanged since the 1960s, were urgently needed, but government legislation is not planned until after 2015.

"There is a great deal of work happening to respond to the challenges the CCC has set out," said environment minister Richard Benyon. "Just this week I've seen in the New Forest the work to join up different landscapes [and] we have set out in the water bill how we want to reform the water industry. However, this is not just a role for government; businesses, communities and local councils all need to play their part."

Ceris Jones, NFU climate change adviser, said the water needed to grow food could be squeezed further by the competing needs of a growing population and taking less water from rivers to protect wildlife as summers get drier. She agreed more reservoirs are needed, but said farmers needed financial incentives to build them and urged more research on drought tolerant crops and efficient irrigation.

The CCC report, published on Wednesday, spells out the challenges for agriculture and other land uses from the extreme weather expected from global warming. It warned that carbon-rich soils on which many crops depend are being washed or blown away in places. "This comes down to the more careful stewardship of soils by farmers," said Krebs.

The report also warned that the retreat from coastlines as sea level rises must be speeded up five-fold or risk serious flooding. Over half of England's coastline is protected by sea walls, but rising seas are drowning the mudflats and salt marshes between the walls and the sea. This reduction leaves the sea walls more vulnerable to storms and the government has already agreed that 10% of sea walls must be moved inland by 2030. However, the current rate of realignment is far too slow, found the report.

The degradation of peatlands, which store huge amounts of carbon and act as sponges to reduce flooding, must be reversed, the report concluded. The damage is caused by burning, to encourage new heather growth for grouse shoots, and overgrazing by sheep, but Krebs said the management of peatland must be rebalanced towards the wider public benefits.

The report also warns the proportion of England's most important wildlife sites that are in good condition has fallen from 42% to 37% in the last decade and that the majority of native species are suffering long term population declines. However, more sites now have action plans for improvement, noted Krebs. "If the action plans are delivered, things could get much better. It boils down to whether they are just a gleam in someone's eye."

Friends of the Earth's Andrew Pendleton said: "Climate change poses a devastating threat to our environment, food supplies and security, which could trigger future economic crises. Urgent government measures are needed."