David Cameron has promised a fight "for the future of our NHS" during a visit to an Oxfordshire hospital.
Mr Cameron says district hospitals are under threat
The Tory leader was visiting the Horton General Hospital in Banbury where there are proposals to downgrade its maternity unit to a midwifery unit.
He said there would be a "bare knuckle fight" with the government over the future of district hospitals.
Prime Minister Gordon Brown has denied plans for a mass closure of hospitals and says the Tories are scaremongering.
Mr Cameron has pledged to put the NHS at the centre of his agenda.
He visited the Horton General on Friday, where Oxford Radcliffe Hospitals NHS Trust is undertaking a "performance improvement and cost reduction programme".
It proposes allowing only midwife-led deliveries at the hospital in future - any deliveries needing an obstetrician would be done at Oxford's John Radcliffe Hospital about 30 miles away.
The trust says the hospital's maternity unit, which delivers less than 1,600 babies a year, is the ninth smallest in England and faces staffing and cost pressures.
Mr Cameron said the situation was commonplace around the country.
He said: "The government's new health adviser, Professor Ara Darzi, says that district general hospitals are over. I completely disagree. I promise Gordon Brown that there will be a fight for the future of our NHS."
In a statement the trust said: "The Horton General Hospital is highly regarded and much valued by our local community, and today's visit was a chance to find out about the challenges affecting smaller hospitals throughout the country."
Mr Cameron says 40 maternity units and 90 accident and emergency units are "under threat".
The Department of Health has said there are 13 NHS trusts where services are the subject of a review or consultation - which may include more than one hospital each. It has not produced a list of affected maternity units.
In July Mr Brown accused the Conservatives of running a "scare campaign" about the government's NHS review.
The prime minister said there would be improved specialist care and that the Conservatives would "not match" Labour on spending commitments.