A lack of hospital capacity is being blamed for regular blockages in emergency departments, after data released exclusively to HSJ revealed patients have already spent hundreds of hours stuck outside hospitals in ambulances this year.

London Ambulance Service Trust operational data shows that for the first seven weeks of 2012, there were 673 incidents – known as “black breaches” – where a patient in an ambulance could not be admitted or more than an hour.

As HSJ revealed last week, NHS London is concerned about emergency service performance and reported its worst performance of 2011-12 during February. It is the first time data showing the picture for every trust in the capital has been compiled and made public. However, direct comparisons are not possible with data from last year, as different information was collected in 2010-11.

The problem is particularly severe at some hospital sites. At Queen’s Hospital, run by Barking, Havering and Redbridge University Hospitals Trust, there were 186 reported black breaches between the beginning of the year and 19 February.

North West London Hospitals Trust’s Northwick Park site had 105 black breaches and Lewisham University Hospital Trust 84 over the same period.

There were no black breaches at 10 of London’s 33 accident and emergency units.

NHS London has stressed that this winter has been relatively mild in terms of flu and vomiting viruses and icy conditions, although an increase during February has put pressure on acute services.

A spokeswoman for the strategic health authority said Northwick Park Hospital had seen a 10 per cent increase in A&E attendances over the past three years and had a “lack of bed capacity”. Bed capacity was also identified as an issue at Queen’s.

Northwick Park was last week earmarked to receive £21m of Department of Health cash to pay for a new A&E department.

A Lewisham spokesman said ambulance traffic had seen an “unprecedented” increase and “emergency department facilities are currently being upgraded”. The work is due to be finished in April.

Derek Hicks, fellow of the College of Emergency Medicine, told HSJ delayed admissions were a problem nationwide.

He said: “A relatively small increase in ambulance numbers is resulting in a disproportionate increase in delays.”

Andrew Frazer, chair of the Royal College of Nursing’s emergency care association, said: “Across Britain we have cut beds in order to save money. We did that with the mantra that we will get more efficient but that hasn’t matched up.”