The NHS has a "significant problem" with ambulances queuing at A&E departments waiting to drop off patients, a senior official said today.
Dame Barbara Hakin, chief operating officer of NHS England, was answering accusations that ambulances are "stacked outside A&E departments like planes".
She told a hearing of the House of Commons Public Accounts Committee that everything was being done to get ambulances back in service as quickly as possible.
She said: "To get ambulances back on the road then we have to improve what's happening in the A&E department. The A&E department has to be able to accept the patients otherwise we will compromise patient safety."
Labour MP Nick Smith said: "Every weekend evening I hear tales of ambulances stacked outside A&E departments like planes over Heathrow. That isn't good enough. What are you doing about it?"
Figures obtained by the committee show that in 2011/12, 2,061 patients waited more than two hours to be transferred to A&E from ambulances, going up to 3,424 in 2012/13. In some places, patients waited more than five hours to be transferred from an ambulance to A&E.
Dame Barbara said £250m of cash had been given to hospitals where the problem was at its worst.
"They came up with a range of things to improve A&E which they are now implementing," she said.
"We are seeing some changes in some of the hospitals although in other hospitals which have had money we haven't yet seen an improvement in the A&E standards on what's going on.
"We are doing everything we can to get ambulances on the road but we are coping with a system that's extremely busy indeed."
Dame Barbara disputed the individual figures given to the committee but added: "I think we have to accept we have a significant problem with ambulances not being able to turn around from acute trusts quickly."
The committee is investigating the reasons behind rising A&E attendances in England.
Dame Barbara told the committee that poor access to GPs increased attendances at A&E.
She said: "What we do know is that poor general practice - part of it is because it's generally poor, they don't look after patients as well in the longer term and that can lead to avoidable admissions - but certainly if patients can't get access to a GP then they tend to go to A&E."
She said it was "very difficult" to be certain whether the scrapping of a target by the Government - which said patients should be seen by a GP within 48 hours -had contributed to a rise in attendances.
"We do know that we've got to make a big improvement in some places. Lots of general practice is very good and there is good access. But in some places the standard of general practice is too poor, the access to general practice is not immediate enough."
She said this was often in urban areas, and as a result it was "unsurprising" that patients went to A&E departments.
And Dame Barbara said decisions to close walk-in centres - some of which were next to A&E departments - were made at a local level.
"In terms of locating services near to A&E or having good access to out-of-hours or having GPs in A&E....it certainly helps take the pressure off A&E departments and some walk-in centres where co-located.
"Some walk-in centres have closed. Clinical commissioning groups have taken over the responsibility of commissioning walk-in centres and some determined it wasn't the best use of their money."