Britain is facing the threat of a major cash crisis after union chiefs warned a strike by almost 1,500 workers could leave ATMs empty and airports without foreign currency. Trade union GMB has sounded the alarm over a bitter dispute involving cash handlers employed by G4S and Loomis, two of the biggest companies responsible for transporting money around the country.

The row centres on pay freezes, with workers now threatening industrial action. Union bosses fear any walkout could have devastating consequences for the UK's cash system, especially if it coincides with a cyberattack or major outage affecting electronic payments. GMB national officer Eamon O'Hearn has written to the Bank of England urging it to intervene before the situation spirals into a national emergency. He warned that the cash industry is "reaching a point where it is no longer sustainable", posing "a material threat to theUK economy, national security and the livelihoods of thousands of workers, their families and communities at large".

The union said ATMs could run dry, while airports may struggle to provide foreign currency for holidaymakers and business travellers.

There are also fears that vulnerable people who rely heavily on cash would be hit hardest if digital payment systems were disrupted.

In a separate letter to Security Minister Dan Jarvis, Mr O'Hearn warned about the resilience of Britain's cash system in the event of another electronic payment crash.

He said: "This raises serious questions about the resilience of the UK cash system, especially in the likely event of another inevitable electronic payment system outage or even cyberattack.

"We know the Government shares some of these concerns as witnessed by present efforts to develop alternative payment systems, however these outages and attacks impact the most vulnerable in our communities who disproportionately experience the digital divide."

The GMB also warned that cash handling and transport workers are facing increasing threats from organised gangs and attacks while carrying out their duties.

Almost 1,500 cash handlers employed by G4S and Loomis are involved in the dispute, with the union warning that Britain's entire cash handling network is coming under strain.

GMB said the industry has become increasingly fragile at a time when cash transport workers face a growing risk of being targeted by organised criminal gangs.

The union said it has spent years trying to professionalise the industry and improve standards to protect staff and stop criminal elements infiltrating the sector.

Mr O'Hearn wrote: "The health of the industry is fundamentally under threat. GMB has worked tirelessly over the years to ensure that the cash - and wider guarding - industry were professionalised and standards raised to protect workers and ensure that criminal elements were kept out."

High-stakes talks between the union and the companies are taking place this week in a bid to avoid strike action. The Bank of England said it would respond to the union's concerns.

A G4S Cash Solutions UK spokeswoman said: "We are continuing to engage with the GMB and hope to reach an amicable agreement without the need for industrial action."

Loomis declined to comment while the dispute remains ongoing.