The number of people waiting for NHS hospital treatment has risen by more than 300,000 under the coalition, amid concern that NHS rationing is forcing patients to wait longer for operations.

NHS data shows that 2.88 million people in England were waiting for consultant-led treatment in December 2013, up by 310,000 or 12% on May 2010.

"It's worrying that the number of extra people waiting is that high. That 310,000 is a huge number of people who are waiting for what is quite often life-changing surgery," said Katherine Murphy, chief executive of the Patients Association.

Patients were facing delays before having a cataract removal, hip or knee replacement or other operation, she said. "Waiting lists are growing because of NHS rationing and because of financial pressures and the NHS's efficiency drive."

NHS England's annual report for 2012/13 on the referral to treatment (RTT) programme, under which patients are meant to be treated within 18 weeks of being referred, usually by their GP, makes clear that around 250,000 more patients were waiting to start treatment during 2013 than during 2012.

The 18-week target is politically important and is seen as having the same significance as the requirement for hospitals to treat 95% of arrivals within four hours. Overall, the targets for treating admitted or non-admitted patients were met in 2012/13, the report says.

Labour claimed that lengthening waiting lists showed the NHS was going backwards as a result of what it called government mismanagement. "David Cameron promised to keep waiting times low but these figures show he has broken his promise", said Andy Burnham, the shadow health secretary.

NHS England said the ageing population and growing number of people with one or more long-term conditions was creating more demand for its services, and it was treating more people.

The Conservatives highlighted the fact that fewer people were waiting 18, 26 or 52 weeks for treatment than at any time under Labour, and the NHS was performing 475,000 more operations per year than before May 2010, even though the population was growing.

"These figures are really a testament to the hard work of doctors and nurses in meeting increased patient demand while keeping waiting times lower than Labour ever managed", said a spokesman.

The Department of Health said: "Over a million NHS patients start treatment with a consultant each month and the overwhelming majority are seen and treated within 18 weeks."