One third of NHS contracts have gone to private sector providers since health reforms were introduced, figures obtained by the British Medical Journal (BMJ) show.
Of 3,494 contracts awarded by 182 of England's clinical commissioning groups (CCGs) since they came into being in April 2013 under the Health and Social Care Act, which gave clinicians greater control over funding, almost half went to non-NHS providers.
Among these more than 300 went to voluntary and social enterprise sector providers while 100 were awarded to others such as joint ventures or local authorities, according to figures released to the BMJ under the Freedom of Information Act.
Some 33% (1,149 contracts) were awarded to private sector providers, the journal states, claiming campaigners said the findings "provided further evidence that the Government's reforms are gradually accelerating the privatisation of the NHS".
A Department of Health spokesperson told the BBC: "Official NHS accounts show that use of the private sector amounts to only six pence in every pound the NHS spends, slowing the rate of increase to just one penny since May 2010.
"Charities, social enterprises and other providers of healthcare play an important role in the NHS, as they have done for many years."
Private sector providers were most successful at winning contracts awarded via competitive tender, the BMJ said, being awarded 41% compared to 30% won by NHS providers.
The BMJ stated that private firms were more likely to win smaller contracts on an any qualified provider basis.
The journal said it found concerns among health professionals about "fragmentation of care and a lack of transparency over where NHS funds were being spent".
The journal said it has requested details for all contracts awarded, to gauge where money was being spent but said many CCGs were " unable or unwilling to provide figures".
The value of 1,349 contracts was revealed though, coming in at £10 billion in total. NHS providers were awarded the bulk of those, valued at £8.5 billion, with private sector providers accounting for 5% (£490 million).
Steve Kell, GP and co-chairman of the NHS Clinical Commissioners representative group, said: "CCGs are trying to develop a sense of joint responsibility for populations, so people in the hospital are thinking beyond their hospital walls to look at nursing home quality, and anything that affects our patients. It's about working together."
Meanwhile NHS spending on consultancy has doubled since 2010, a Freedom of Information request by David Oliver, a former clinical director at the Department of Health, also published in the BMJ shows.
Mr Oliver said the figure had shot up from £313 million to £640 million in the past four years, and claimed it would be enough to run three medium sized hospitals or employ about 2,000 extra nurses.
"In times of war, arms dealers, rebuilders, and racketeers profit from the chaos," he said, calling for " management consultants to face the same transparency and accountability as the rest of us".