Measures were being taken to ensure robust plans are in place for delivery of the most critical services - such as emergency care, end-of-life care and renal dialysis.
But the organisation warned some patients and relatives would experience delays and distress.
On the issue of safety, Dean Royles, the director of NHS Employers, said: "I want to be clear that the NHS's overriding priority on the day of action will be the safety of our patients. We believe trade unions share this priority and they have said they will work closely with the NHS at a local level to maximise patient safety and the delivery of emergency services.
"There is no room for complacency on this issue, but the indications are that these discussions are going well.
"Trusts have been developing plans on the areas of care likely to be affected and developing contingency plans with trade unions and neighbouring NHS organisations."
On the overall impact of the strike, Royles said: "Many services will be working in much the same way they do at a weekend or on a bank holiday. There may often be more staff in work than there would be on bank holiday, given a number of trade unions are not balloting and a number of staff are not union members. Some operations, out-patient appointments, tests and follow up appointments will be postponed and organisations will be informing patients of delays and postponements.
"Trusts will bend over backwards to recover the position but may also see some people staying slightly longer in hospital."
Royles said a key uncertainty was the impact on the NHS of strike action in other parts of the public sector, particularly schools and transport. Trusts would be communicating with staff about the possible need for extra childcare on the day.
He added trusts would also be discussing picketing with trades unions. They would of course seek to allow protests to take place but to ensure there is no unnecessary disruption to services or undue distress to patients accessing services.
On the industrial relations climate, Royles said: "A strike is still very unusual in the NHS and we recognise the anxieties of staff over pensions. But we need to remember that staff and patients have benefited hugely from the positive industrial relations climate that has existed in the NHS for a generation.
"I know that NHS organisations and trades unions - and indeed our patients - will not want that to be lost. We do not want to wake up on December 1 and find that relationships have been damaged and that patient care suffers as a result. We have got to sustain a positive working environment in the interests of everyone."