Hundreds of thousands of patients in need of emergency care are being left at the doors of A&E in ambulances because hospitals are too full, Labour has said.
New figures obtained by the party show that last year almost 300,000 patients in England were caught in queues of ambulances outside emergency departments.
In 2013/14, 279,207 ambulances were delayed for more than 30 minutes and a further 30,601 faced waits longer than one hour, a Labour said.
Freedom of Information requests conducted on all 10 ambulance trusts in England also revealed that some patients were forced to wait in these queues for hours on end.
A patient in the West Midlands was forced to wait for eight hours and 11 minutes, the party found. Another in the South West waited for seven and a half hours.
Labour said that the "handover delays" occur when ambulance crews cannot transfer a patient to the A&E department because of staff or bed shortages.
Labour's shadow health minister Jamie Reed said: "Under David Cameron, hospitals are full to bursting and he's forcing ambulances to queue at the doors for hours on end.
"Thousands of vulnerable people, many of them elderly and frightened, are being wrongly held in the backs of ambulances because hospitals don't have the space. And yet ministers deny that A&E is in crisis.
"People know from their own experience that the NHS is heading downhill on David Cameron's watch. It is clear the Tories can't be trusted with it."
A Department of Health spokesman said: "People rely on ambulances coming quickly in life and death situations and long handovers are completely unacceptable.
"We're already making good progress in reducing the number of patients waiting for 30 minutes or longer - down by almost a third last winter - though there's always more the NHS can do.
"We are providing extra support, including £28 million for ambulances from funds already given to the NHS this year, to keep services sustainable year-round. In the long-term, we want to reduce demand by looking after people better in the community."
An NHS England spokeswoman said: "The figures published for the winter of 2013/14 show that the number of ambulance handover delays were actually down on the previous year by 30%.
"We know that demands on ambulances are rising every year, and we're allocating a further £28 million to ambulance providers to help them deal with these pressures.
"However in some cases it may be the right thing to do for a patient to be cared for in the ambulance before transfer to stabilise their condition.
"One of the aims of our Urgent and Emergency Care Review is to capitalise on the skills and abilities of paramedics and the wider workforce so that ambulances can become more of a mobile treatment service, rather than just a transport service."