Changes to the way assaults on NHS staff are recorded have been criticised by the Royal College of Nursing (RCN).
The RCN expressed concern that the Government intends to rely on an annual survey that health service workers complete on an optional basis for information, rather than replace the collection of assault figures which used to be undertaken by the now scrapped body NHS Protect.
The criticism comes as MPs debate a backbench Bill on Friday which aims to toughen penalties for attacks on NHS staff and other emergency workers.
The proposed legislation, which has Government backing, seeks to double the maximum sentence for common assault from six months to a year if committed against an emergency worker while on duty.
The RCN claims that changes to the way assault figures are collated would not provide a full picture, and would be unable to distinguish between intentional attacks and those related to a patient's medical condition.
The nursing union said a survey of its members found that 56% had experienced physical or verbal abuse from patients, and 63% from patients' relatives, or other members of the public.
The RCN said that final figures from NHS Protect showed a 4% rise in physical assaults against healthcare workers in England, from 67,864 in 2014/15 to 70,555 in 2015/16.
The body found that only 10% of physical assaults, unrelated to a medical condition such as a mental health problem or dementia, resulted in criminal sanctions, the RCN said.
The RCN's Kim Sunley said the new system for collating assault information was not thorough enough.
She said: "This creates a dangerous blind spot for ministers hoping to tackle the increasing number of assaults in the NHS. It is totally inadequate to rely on optional surveys, especially if the law is being tightened.
"The official body, before it was disbanded, warned ministers the level of assaults was rising. It should not have been removed and the Government must take their role more seriously.
"This Bill represents a vital step towards achieving that, but without the ability to fully monitor the figures, it will be difficult to quantify the scale of the problem, or the effectiveness of any new law."
A Government spokesperson said: "NHS staff work incredibly hard in a high-pressure environment - it is completely unacceptable for them to be subject to aggression or violence and employers should have no hesitation in involving the police.
"We continue to collect data on physical assaults against NHS staff through the annual NHS staff survey, with trusts also collecting data at a local level, and we are making crucial legal changes to ensure those who are violent face the full force of the law."
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