England will require frontline health workers to be vaccinated by April.

All frontline health workers in England will have to be vaccinated against Covid-19 by next spring to keep their jobs, the country’s health secretary said on Tuesday, a move that employers and trade unions warned could aggravate staff shortages.

“We must avoid preventable harm and protect patients in the N.H.S., protect colleagues in the N.H.S. and, of course, protect the N.H.S. itself,” Sajid Javid, Britain’s health secretary, told Parliament, referring to the National Health Service. He added that about 90 percent of the service’s workers had received at least two vaccine doses.

The measure, which is subject to parliamentary approval, is due to come into force on April 1. Exemptions will be available for people who are medically prevented from receiving vaccines and for health workers who have no face-to-face contact with patients.

That time frame is intended to ensure that workers who do not want to be vaccinated remain in their jobs during the winter, when a strain on the country’s overstretched health service is likely to be particularly acute.

Mr. Javid said he had decided against making flu vaccination compulsory for the moment.

England’s heath service employs around 1.3 million workers, though not all are in frontline positions. About 80,000 to 100,000 N.H.S. workers in the country remain unvaccinated against Covid, according to Chris Hopson, the chief executive of N.H.S. Providers, a membership organization for the N.H.S. hospital, mental health, community and ambulance services.

Scotland, Wales and Northern Ireland can make their own decisions on the issue, and so far have not put forward proposals.

Compulsory vaccination is also a sensitive issue in the United States, and the Biden administration is pushing to introduce a coronavirus vaccine mandate for large businesses.

In Britain, one main concern is that people reluctant to be vaccinated will quit their jobs and worsen staff shortages within a health service that is under acute strain and that expects more pressure as winter sets in.

People working in care homes are required to be vaccinated as of Thursday. Some are thought to have quit their jobs and opted to work instead in the National Health Service.

Mr. Hopson said that although there was a risk that unvaccinated workers could infect patients and colleagues, staff shortages also posed a danger to the public’s health.

“The problem for both social care and the N.H.S. is that we run these systems incredibly hot on very very fine margins,” he told the BBC. “Both of us have got around 90,000 to 100,000 vacancies.”

“We are completely reliant on our staff to currently work extra shifts in order to do the work that needs to be done,” he added, “so losing significant numbers of staff, particularly given the pressures both systems are under at the moment, is a real, real problem.”

Trade unions issued more blunt warnings.

“Bulldozing this vaccine will exacerbate the already crushing staffing crisis we face across the N.H.S. and ambulance services,” said Rachel Harrison, a national officer at the GMB union. “Both are operating under extreme pressures, after a decade of austerity and cuts, with an exhausted and demoralized work force who are fearful of what is to come as we head through winter.”