The organisation said a lack of opportunity to plan could increase the potential of distress for patients and exacerbate disruption, which could include delays in hospital tests and discharges from hospital.
Dean Royles (pictured), the director of NHS Employers, said: "I recognise that the union is disenamoured about pensions but patients must be our first priority.
"We have serious concerns that the NHS is not being given the opportunity to plan effectively for the strikes on 10 May. At times of industrial action, the most important thing for patient safety is planning.
"Employers are telling us they have not yet received written notification on which staff will be asked to withdraw their labour with action scheduled to take place in less than two weeks.
"Unions must by law do this no later than seven days before a strike. But this is healthcare we are talking about. Employers are having to rely on patchy information from local representatives but this is not enough on which to take significant decisions about patient care.
"Remember that trusts need to get the information, speak to staff and managers and then decide if they need to cancel, postpone or rearrange any patient appointments and let the patients know, all within seven days.
"Of course the NHS will work hard and cope but we could be so much clearer to patients and maximise safety with written information from Unite. Even in a well planned strike, there will be distress and disruption to patients.
"This is bad enough but problems will be exacerbated with every day that passes without reliable information. Employers will keep focussed on minimising disruption and ensuring that the safety of patients is put first."
