Around 20 NHS staff every day in Wales complain of abuse
Staff working in Welsh hospitals will not have extra legal protection from violence and abuse offered to their English colleagues, it has emerged.
The Ministry of Justice said Welsh ministers rejected the chance to extend a new law to Wales, although it is discussing the issue with the Wales Office and Department of Health.
The assembly government said it was developing its own policies.
Cross-bench peer Baroness Ilora Finlay described the situation as "madness".
More than 20 NHS staff in Wales complain of physical or mental abuse in a typical day. The UK Government is introducing a new offence of creating a disturbance or nuisance on NHS premises to help hospitals protect their staff.
But the assembly government told the Ministry of Justice it did not want the new law to extend to Wales.
It has instead set up its own policies to protect staff, although it does not have the power to change criminal law.
It said a group led by a representative of the Royal College of Nursing is expected to report imminently on what it describes as "actions designed to meet Welsh needs and circumstances."
That is unlikely to pacify 20 MPs and peers who say it is absurd that Welsh workers will not receive the same legal protection as their English counterparts.
Former Welsh Secretary Alun Michael blames the gap on a misunderstanding of devolution within Whitehall.
Ministers have pledged 'zero tolerance' of aggression
He has tabled a Commons motion to try to put pressure on ministers to change the bill.
Baroness Ilora Finlay first brought attention to the situation when she tried to have the extra legal protection for NHS staff extended to healthcare workers in hospices and working in the community.
She said: "It seems madness to not to take on protection, potentially, for healthcare workers, that we have a chance to get this legislation in place to apply to Wales.
"The Welsh Assembly Government cannot legislate on criminal justice issues so, effectively, if they've got it wrong in not taking this opportunity, they're going to have to wait many years to have another opportunity to protect healthcare workers."
"People need to be aware, if healthcare workers in Wales do not have equal protection with healthcare workers in England, we will not be getting the best of frontline healthcare workers, particularly accident and emergency, and those kind of people wanting to come back to Wales."
Health Minister Edwina Hart said a working party was developing practical measures to protect NHS staff in Wales from abusive or violent patients and relatives.
She said there were already enough legal powers to deal with violence in Welsh hospitals.
"We've got to progress things in a practical way and we've got to get solutions to this and I'm not certain that what [the MPs and peers] are doing is going to make any difference," she said.
"You've only got to look at the laws that have come about over the years. There's been a lot of legisation [about] guns, knives and everything, and what real progress has there been made?
"What I'm looking at is how I can get better relationships going with the police - the police have been marvellous - what we can do with the CPS and how we can get the reality of change."
A Ministry of Justice spokesperson said: "Health is a devolved matter in Wales.
"We are discussing the issue of the extension of this provision to NHS premises in Wales with the Wales Office and the Department of Health."