Hospitals have now stepped up security, carried out drills and put additional precautionary measures in place, NHS chiefs told The Yorkshire Post.
It comes after counter-terror police hunting potential accomplices to the Manchester suicide bombing carried out fresh raids and made a series of arrests.
Eight suspects aged between 18 and 38 are in custody in connection with the bombing, counter-terror detectives said yesterday.
Twenty-two people were killed in the blast, including seven children, on Monday night while on Friday NHS England said 66 injured were still being treated across eight hospitals, 23 of whom are in critical care.
In response to the threat level being raised, NHS England wrote to trusts to ensure robust protocols for security were put in place.
Trusts across the county have stepped up security and, in some places, hospital sites are under high alert procedures overnight.
They include Bradford Teaching Hospitals NHS Foundation Trust, which is enforcing extra measures at hospital sites from 9pm - restricting access except for ID holders or ambulances.
“To provide reassurance as a precaution we are strengthening our security arrangements,” a spokesman for the trust said.
“We are asking staff to be extra vigilant.”
Elesewhere, Mid Yorkshire Hospitals NHS Trust said staff were trained in how to respond in the event of a terror attack.
Trusts in Leeds, Sheffield and Hull have also increased security.
Suzanne Hinchliffe, deputy chief executive at Leeds Teaching Hospitals, said: “NHS England has written to all NHS hospitals following the Prime Minister’s announcement that the country’s security alert level has been raised to critical. We have well developed emergency plans which are regularly rehearsed.”
Leeds General Infirmary, Hull Royal Infirmary and Sheffield’s Northern General Hospital and Children’s Hospital are among 27 Major Trauma Centres now on standby to prepare for the possibility of further attacks.
Greater Manchester Police recorded a spike in hate crime following Monday’s bombing.
A total of 56 hate crimes were recorded in the city on Wednesday, a significant rise on the 28 reported on Monday, the force’s chief constable Ian Hopkins said.