Ministers have been accused of “selling off the NHS family silver” after figures revealed that the amount of health service land being earmarked for sale to private developers is soaring.

The NHS is seeking buyers for 718 different plots of land or buildings it owns across England, prompting fears that underfunding has forced cash-strapped NHS trusts to dispose of vital assets.

The total of 718 sites represents a 72% rise on the 418 plots the NHS deemed as surplus to requirements two years ago.

The number of sites on the market that NHS bosses say are currently being used for clinical or medical purposes is also rising fast, from 117 last year to 140 – almost one in five of the total.

Seven of the top 10 sites with the highest value fall into that category. They include a part of Heatherwood hospital in Ascot, Berkshire – which is used by patients from Theresa May’s nearby Maidenhead constituency – valued at £35m, and part of the site of Birmingham’s City hospital (£18.8m).

Labour said the figures, contained in the NHS’s annual register of land for sale, showed that hospitals were being forced into a “firesale of assets” after years of being starved of resources while the government had restricted annual budget rises to 1% since 2010.

“Hospitals are struggling to cope with cutbacks from the Tories. The answer should be a serious long-term government-funded investment plan and not selling off the NHS’s family silver,” said Jonathan Ashworth, the shadow health secretary.

Last year a government-commissioned report by Sir Robert Naylor, a former University College London hospital chief executive, said the NHS could raise £6bn from taking a more “commercial approach” to disposing of land.

Ministers approve of the growing selloffs, which they say will help generate receipts that NHS trusts can then use to redevelop their facilities and build homes for staff.

The British Medical Association, which represents doctors, voiced unease. It said the selloffs were short-sighted and could leave hospitals with too little space to expand in future.

Dr Chaand Nagpaul, the chair of the BMA council, said: “These figure show a staggering increase in sale of NHS land in the last two years. This begs serious questions as to the reason for this surge. Was this land actually surplus or are these sales being used to plug financial deficits in hospital trusts as a result of a decade of underfunding?

“It is vital to safeguard the sale of NHS land and estate from perverse short-term financial incentives, and which may result in a reduction in estate and facilities that is insufficient to meet the future needs of patients. These figures demand scrutiny. Selling land shouldn’t be a way for the health service to make up for austerity-era cuts – especially if it could come at the expense of patient care.”

The total amount of land involved in the NHS asset sale has grown from 545.7 hectares (1,348 acres) in 2015-16 to 1,332 hectares in 2016-17 and 1,749.4 hectares last year, according to research undertaken by the House of Commons library for Ashworth.

The Department of Health and Social Care defended the rise in sales. A spokesperson said: “As part of the long-term plan for the NHS we are committed to making taxpayers’ money go further, including getting the best use out of the land and buildings the NHS owns.

“We are helping trusts dispose of surplus land or buildings so that money is saved and spent instead on improving patient care, whilst freeing up space for much needed new homes, including for NHS staff.”