Emergency 999 call handlers have gone on strike with colleagues from BT - which runs their phone lines - in a row over pay.
Hundreds of operators were set to walk out on Thursday, with the union behind the action warning there could be disruption to emergency services as a result.
The Communication Workers Union said it was the first time 999 call operators had gone on strike.
In previous BT strikes, it had agreed to exempt these workers.
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But the union said this would not be the case on Thursday following “widespread outrage at the company’s refusal to negotiate with union representatives”.
More than 500 operators in 999 call centres were expected to join the strike.
“This decision was not taken lightly, but our union’s repeated attempts to initiate discussions was declined by a management who clearly believe they are above negotiating a fair deal for people who make massive profits for them,” Dave Ward, the CWU general secretary, said.
“999 operators are using food banks, they’re worried about the cost of living and are being stretched to the limit. Goodwill won’t pay the bills, and vital services are now being hampered because of corporate greed.”
Joyce Stevenson, a 999 call handler going on strike, said the decision to walk out “was not easy”. But she said colleages had been “pushed to the limit” with soaring inflation and paying bills had become “a huge worry” in a video posted on CWU’s social media.
Around 30,000 Openreach engineers and 10,000 BT call centre workers have already taken industrial action, and further strikes are planned for October 10, 20 and 24.
The union said the strikes are in opposition to BT Group’s imposition of an “incredibly low” flat-rate pay rise of £1,500, which it described as a “dramatic real-terms pay cut” because of soaring inflation.
“BT management clearly has no appreciation of its workforce, or their social value, but this union does, and we will keep on fighting this company’s attempt to plunge its workers into even further hardship,” Mr Ward added.
A BT Group spokesperson said it respected the right of its workers to strike, but the company was “profoundly disappointed” the union was “prepared to take this reckless course of action by including 999 services” in industrial action.
“We will do whatever it takes to protect 999 services – redeploying our people to the most important priority is a normal part of BT Group operations,” they said.
“We made the best pay award we could in April and we have held discussions with the CWU to find a way forward from here. In the meantime, we will continue to work to minimise any disruption and keep our customers and the country connected”.
Additional reporting by Press Association