Mr Lansley was speaking in the wake of a Royal College of Nursing (RCN) study that warned more than 60,000 frontline jobs in the NHS, including those of nurses, were at risk because of spending cuts, with almost half already gone.
He was laughed at by members of the audience at the RCN annual conference after saying nurses should tell superiors if staffing levels were not safe.
And some in the crowd shouted “liar” after the Health Secretary claimed clinical staffing levels had increased on his watch.
Mr Lansley told the conference in a question and answer session: “There are places across the country where from time to time, the Care Quality Commission on our behalf, as the inspector, finds that staffing levels are not safe.
“Current warning notices are in place that staffing levels were thought not to be safe at Dewsbury, at Leeds, at Lancaster, at Mid Staffordshire, at Pembury and at Queen’s Hospital in Romford. That can happen and we do need to identify it.
“If any of you have a view that staffing levels are literally not safe for patients I think part of your professional responsibility is to say that. Part of the responsibility of nursing directors and trust boards is to listen to what you are saying.
“Across the whole of the NHS we have seen staffing levels reduce. But clinical staffing levels overall have gone up by nearly 4,000. The number of qualified nurses has gone down by nearly 3,000 in two years in England but those are decisions made by trust boards. They have actually increased the number of doctors by nearly 4,000.”
The RCN said 61,000 posts were at risk of being slashed across the health service, including nursing and other jobs, with 26,000 already lost in the two years to April.
The loss of so many jobs showed the “weakness” of Government pledges to protect the front line, according to the RCN.
But Mr Lansley insisted it is managers and administrators who have lost their jobs rather than clinical staff.
He said: “The number of staff in the NHS has gone down but actually the number of clinical staff has gone up.
“The number of managers and administrators has gone down. We do need to sustain that progress to get resources to the front line to ensure that they are owned and used by clinicians.
“Clinical leadership is at the heart of the process of reform across the NHS.”
The claims were dismissed as “nonsense” by RCN general secretary Peter Carter.
Speaking to journalists, he said: “All this nonsense that there is more clinical staff now than there were a few years ago is simply not true.”
And Mr Lansley was laughed at for saying nurses should take more responsibility because in the past whistleblowers have been ignored or persuaded not to speak out, according to Dr Carter.
He said: “What we have got to remember is in many hospitals where there have been scandals people were speaking out but they have either been ignored or been leant on.
“If you are not able to speak out, come to us and we will help you.”
The RCN study also showed that community services, covering district and mental health nurses and those who visit patients in their own homes, were being “overburdened”.
Cuts and underinvestment risked a “revolving door” for patients, who are discharged from hospital only to find there is no support in the community so have to be readmitted to hospital, it was warned.
Fewer than one in 10 of 2,600 community nurses polled by the RCN said they had enough time to meet the needs of their patients, while nine out of 10 revealed that their caseload had increased in the past year.
The RCN said planned job cuts included more than 400 in Greater Glasgow and Clyde, a similar number in Sandwell and West Birmingham, 675 in Blackpool and a 23% reduction in staffing numbers in south London up to 2015.
The coalition was urged to take the “stark” figures seriously but this morning both Health Minister Simon Burns and Deputy Prime Minister Nick Clegg said the Government did not recognise the figures.
Mr Burns said: “Official statistics show that there are only 450 fewer qualified nursing staff in England than in September 2009, while the number of managers has been slashed by 15%.
“The number of nurses to beds in hospitals is going up and in 2011/12 we expect to train just over 2,300 community nurses and health visitors - double the number of places last year.”
But there is “a great deal of unhappiness” among NHS staff, Dr Carter told Mr Lansley at the end of his appearance before RCN delegates.
Dr Carter said: “You really have to be clear - a lot of people in this hall are feeling that out there, by some of their employers, not all, they are not being treated with dignity, they are not being listened to, and they feel they are not being respected.
“Just take it on board, because you’ve had a highly professional and dignified encounter with the RCN, don’t take that as a kind of indication that things are fine. There is a great deal of unhappiness.”