Banks, hotels, ATMs and pubs could face a cash shortage as more than 1,000 G4S workers vote to strike over a real terms pay cut, GMB said.
GMB members at G4S deliver money to companies such as NatWest, Lloyds Santander, Tesco, Asda, Wetherspoons, McDonalds and Travelodge.
Workers have voted to strike with a majority of 91%, on a 59% turnout.
Industrial action could take place as early as the Easter bank holiday, with business and ATMs potentially left without cash, while airports may run out of foreign currency.
Eamon O’Hearn, national officer at GMB, said: “These workers do a difficult and dangerous job – yet the company is only offering them a real terms pay cut.
“It’s no wonder they are willing to strike.
“Now thanks to G4S penny pinching, the public faces an Easter break where businesses and banks run out of cash, potentially causing major disruption.”
A G4S Cash Solutions UK spokesperson said: “We are continuing to engage with the GMB and hope to reach an amicable agreement without the need for industrial action.”