It was the straw that broke the camel's back for Liz Heyer, chief executive of Barnet and Chase Farm Hospitals NHS Trust.

Miss Heyer was just days away from discovering whether she would be sacked following the trust's dismal zero star performance in last year's NHS ratings when another scandal came to light.

The Commission For Health Improvement (CHI) an independent body that monitors clinical care in the NHS had decided to call an investigation after it uncovered a backlog of 2,700 patients waiting for ultrasound tests at Chase Farm Hospital in Enfield. Although all of the patients affected were from the Enfield rather than Barnet area, Miss Heyer resigned from her £101,000-a-year job as head of the trust, quoting a "major system failure for which I accept full responsibility".

What will worry many patients is that without the CHI investigation, we would never have known the extent of the trust's problems. It was unable to shed any light this week on how such a backlog could have gone unnoticed for so long, saying that was for a CHI investigation to establish the facts later this year.

What is more worrying is that the CHI is also looking into a similar backlog of 800 people waiting for ultrasounds that was built up two years ago.

"It is shocking news it's an awful event," admitted Mike Ward, the trust's medical director. "We accept that there was a failure in our systems that allowed this to happen. It is quite vague at the moment, I do not think I can provide you with any more detail at this stage. Obviously the purpose of the CHI investigation is to identify exactly how this happened."

A consultant radiologist at the trust, who was in charge of the ultrasound service, has been suspended and the General Medical Council's Fitness to Practice Directorate notified. The trust would not reveal his name but said he had never worked at Barnet Hospital.

Mr Ward said the backlog came to light in September 2001, but no-one made sure that the number of appointments was increased to deal with it. "We were aware that the number of ultrasound requests received through the trusts has been constantly increasing over the course of the last three years. We didn't seem to register the same sort of pressure with our appointments," he said.

The trust drew up an action plan and all but 400 of the full 3,500 patients have now had their tests. The others will be seen by the end of the month.

"I can categorically say that all the requests for investigations were looked at and designated as non-urgent," said Mr Ward. "I am not in a position to say with total confidence that nothing was missed but we are undergoing a comprehensive review of all these results as they come through and no serious conditions have been missed so far."

The question now is whether the public can have real confidence that other services at the hospital are performing as badly. The episode has done nothing to allay fears that the trust has become an unmanageable organisation. In last September's NHS ratings it was one of only 12 trusts in England and the only one in Greater London to receive zero stars. It was deemed to be 'under-achieving' in two of the nine core target areas waiting times for breast cancer patients and the number of operations cancelled on the day.

It was also 'significantly under-achieving' in a further two outpatient waits and keeping to its agreed budget. A combination of the ratings and the ultrasound backlog prompted this week's decision by Health Secretary Alan Milburn to franchise the trust to NHS managers with a "proven track record".

In the meantime acting chief executive Paul O'Connor, formerly director of operations at King's College Hospitals NHS Trust, has taken charge.

"I don't think the trust has ever been run properly certainly within my living memory," said Dr Jonathan Lubin, of the Derwent Medical Centre, Derwent Crescent, North Finchley, who has been a GP since 1989.

"It is difficult to know why. I think they have got a lot of problems because there doesn't seem to be enough money. Whether that is because they are using it badly or because there isn't enough I am not sure."

Elizabeth Manero, former chairman of Barnet Community Health Council (CHC), believes Barnet Hospital's merger with Chase Farm three years ago is partly to blame for the crisis. The received wisdom in the NHS at the time was that hospital trusts had to get bigger to offer a wider range of services.

"The CHC's position is that the merger was a mistake. Maybe [the ultrasound backlog] was a symptom of that. If managers are worried about their department and what job they are going to do then the nuts and bolts are going to get neglected it's common sense," said Mrs Manero, now chair of London Health Link, the umbrella group for the capital's CHCs.

"It is not a coincidence that another trust which got zero stars Epsom and St Helier was also a trust that merged. Patient issues inevitably get pushed out because you create a very large, complex organisation."

Hendon MP Andrew Dismore also questioned the merger. "Do we need to de-merge the two hospitals? For example, would Barnet be better merging with the Royal Free or Northwick Park? Does that better reflect its catchment area?

"I am certainly not putting that forward as a proposition but it is a question that anybody coming in with a fresh broom should be asking."

Whatever the future for Barnet and Chase Farm Hospitals NHS Trust, there has been little sympathy for the departing Miss Heyer.

Brian Coleman, the Tory Greater London Assembly member for Barnet and Camden, described her resignation as "good riddance to bad rubbish".

Philippa Curran, vice-chair of Barnet's Local Medical Committee, said: "One has to recognise her dedication to her job and the hugely challenging agenda she faced.

"But when evidence like that is produced in one organisation you have to accept that the service is unacceptable."