A British railway ticket office worker has died from Covid-19 after being spat on while she was working at Victoria station in central London.

Belly Mujinga, 47, was working with a colleague when a member of the public assaulted them, spat and coughed over them and said he had the novel coronavirus, said her union, the Transport Salaried Staffs Association (TSSA), in a statement Tuesday.

Mujinga, who had an underlying health condition, was working for Govia Thameslink Railway (GTR) on the station concourse at the time of the incident on March 22.

British Transport Police said in a statement to CNN that an investigation into the incident had been launched.

"Belly and her colleague begged to be let to work from inside the building with a protective barrier between them and the public for the rest of that day," TSSA said in its statement.

"Management said they needed people working outside and sent them back out onto the concourse for the rest of their shift."

Both women went back outside and completed their shift, added the union, but they had no personal protective equipment.

Mujinga had underlying respiratory problems for which she had had an operation, regular hospital appointments and had previously needed to take time off work, according to TSSA.

The union says GTR knew about her condition and, even after the incident, only stood Mujinga down after her doctor called her work around March 25.

Manuel Cortes, TSSA general secretary, accused GTR of not taking Mujinga's assault "seriously enough" and criticized the company.

"As a vulnerable person in the 'at risk' category and her condition known to her employer, there are questions about why GTR didn't stand her down from front line duties early on in this pandemic," Cortes said in the TSSA statement. "There are serious questions about her death, it wasn't inevitable."

GTR is investigating the accusations, it said in a statement to CNN on Tuesday, adding it takes the allegations "extremely seriously."

"The safety of our customers and staff, who are key workers themselves, continues to be front of mind at all times and we follow the latest Government advice," Angie Doll, managing director at Southern Railway and Gatwick Express, said in the statement.

"We are devastated that our dedicated colleague Belly has passed away and our deepest sympathies are with her family, with whom we have been in touch through this very difficult time," she also said.

Both Mujinga and her colleague fell ill from Covid-19 within days of the assault, according to TSSA.

On April 2 -- just 11 days after the incident -- Mujinga was taken to Barnet Hospital, north London, in an ambulance and put on a ventilator.

"Belly died on 5 April, 14 days after she was assaulted at Victoria station," TSSA said.

She leaves behind an 11-year-old daughter and a husband, who last saw her when she was taken away by ambulance, the union added.

"We are shocked and devastated at Belly's death," Cortes said in the TSSA statement. The union said it is taking legal advice on the situation and supporting Mujinga's family and colleagues.

It also reported the incident to the Railways Inspectorate, the safety arm of the Office for Road and Rail (ORR).

An ORR spokesperson told CNN the office is investigating the incident.

TSSA revealed Mujinga's story as the British Government urged some people to return to work if they are not able do so from home, easing some of the restrictions it had put in place to stop the spread of Covid-19.

The union says there is not enough guidance or protection for workers on the front line, such as Mujinga.

"Rather than talking about the easing the lockdown, the government must first ensure that the right precautions and protections have been taken so that more lives are not lost," Cortes said.

"Our rail industry needs to have a very serious look at what tasks are deemed 'essential' and must put protections in place for all our members and our passengers."

The TSSA also called on the government to implement additional measures to compensate frontline workers from the railway industry for their work during these difficult times.