Sixteen upgrades to England's busiest roads could be scrapped because they do not represent value for money, the Whitehall spending watchdog has warned.

The schemes are part of an £11.4 billion investment in motorways and major roads which must be reconsidered by the summer to ensure it is deliverable and affordable, the National Audit Office (NAO) said.

In 2014, the then coalition government announced details of a five-year Road Investment Strategy featuring 112 major schemes to begin 2015-2020, such as smart motorway upgrades and converting single-carriageway A-roads to dual carriageways.

But government-owned Highways England, which is responsible for maintaining and improving major roads, has identified 16 projects which could be cancelled, delayed or redesigned to ensure more efficient use of taxpayers' money, the NAO said.

Neither the DfT nor Highways England confirmed which projects are at risk.

The overall plan includes a tunnel at Stonehenge, upgrading the M1 to a smart motorway around Sheffield and Rotherham, and improving electronic signage, emergency roadside telephones and CCTV cameras on the M11 between Stansted Airport and Cambridge.

AA president Edmund King said drivers across the country will be frustrated to learn that "even after clear promises were made, upgrades to heavily congested roads could be delayed or scrapped altogether".

He went on: "The public should be told which of the 16 schemes have stalled so that they know if they are stuck on a road to nowhere."

Steve Gooding, director of motoring research charity the RAC Foundation, said: "Highways England has had a mountain to climb in gearing up for the scale of work involved, so it shouldn't surprise us that in some respects it is still in the foothills.

"We need Highways England to deliver the enhanced capacity, enhanced maintenance and enhanced safety we all want in a way that gives best value without undue disruption to the millions of road users who rely on the network daily."

The Road Investment Strategy represents a "significant improvement in efficient management" of the road network due to the long-term strategy and improved certainty of funding, which helps Highways England secure lower-cost contracts with suppliers, the NAO said.

But the watchdog concluded that several problems were created by the plans being drawn up in just 17 months in order to publish them before the May 2015 general election. The equivalent rail planning takes around 30 months.

Among the issues highlighted are that the DfT selected projects without knowing enough about whether they represented the best use of taxpayers' money.

Also, some 54 schemes are scheduled to start in 2019/20, which would cause "significant disruption" to motorists, increase prices and put pressure on resources at Highways England, according to the NAO.

Its report stated that by August last year the amount by which forecast capital costs exceeded available funding had reached £841 million.

Highways England chief executive Jim O'Sullivan said: "We are confident we will deliver our capital programme without overspending our budget.

"So far we have delivered nine schemes on time or ahead of schedule, with one of them, the M25 Junction 30, opening in December 2016 six months earlier than planned.

"There are currently 17 schemes in construction and we are contributing £40 million to a further 13 improvements that will unlock the opportunity to create 22,000 new homes and more than 34,000 jobs.

"We will be publishing our delivery plan update in June, laying out the programme for the current year and again we expect to deliver all of the required work."

A DfT spokesman said: "This Government is taking the big decisions for Britain's future, and we are investing a record £15 billion on road schemes which will cut congestion, speed up journeys, and boost the economy across the country.

"As this report acknowledges, we have made significant progress in managing our major roads more efficiently, and we are confident Highways England will deliver safe and reliable roads that deliver value for money for the taxpayer.

"We will consider the findings of this report."