GMB warned against introducing an "Uber for the NHS" and said the government should focus on lifting the public-sector pay cap and investing in the health service.
In his speech to the Tory Party conference yesterday, Health Secretary Jeremy Hunt unveiled the new system, which is to be trialled in 12 NHS trusts.
Mr Hunt told delegates that nurses must be able to "work flexibly, do extra hours at short notice, get paid more quickly when they do and make their own choices on pension contributions."
He announced that the new "flexible working arrangements" would be offered to all NHS staff during this term of Parliament, with 12 trusts piloting the new "app-based flexible working offer" for its staff.
However, the plan was quickly dismissed as a gimmick, with warnings that nurses will be forced into the "gig economy."
GMB national secretary Rehana Azam said: “Public-sector workers are already putting in £11 billion worth of unpaid overtime every year and many of our NHS members already take on bank work to make ends meet.
"The idea that there is some untapped reserve of labour in the NHS that can be unlocked with an app is pure fantasy."
She warned that having overworked and underpaid staff told by an app at the last minute that they are not needed at work would be "the start of a slippery slope.
“Instead of wasting money creating Uber for the NHS, why doesn’t the Health Secretary give all NHS staff the pay rise they desperately need?” she demanded.