Plans to franchise scores of stores by mobile phone giant O2 put hundreds of jobs into uncertainty, a union has warned.
The Communication Workers Union (CWU) reacted with "shock and anger" to an announcement that 82 retail stores will be franchised.
O2 said 120 of its 464 stories were already franchised, so it was continuing with a "tried and trusted" model.
But the CWU said the move was a "huge gamble" in the fiercely competitive mobile market.
Deputy general secretary Andy Kerr said the company was "playing poker" with workers' livelihoods in a move he warned would destabilise the whole network of shops.
He said: "Staff will be wondering whether they have a job and what their future holds as they enter the busy pre-Christmas shopping period.
"O2 shop workers are the lowest-paid staff in the company and are the most vulnerable. Not only will staff feel shocked by this decision, but we believe O2 is taking a huge risk by moving into this model at this time.
"The company is simply hoping that appropriate candidates will come forward as franchisees in response to their advert instead of directly managing their brand and high street sales."
An O2 spokesman said: "We've been operating a successful franchise operation since 2005. Offering a further 82 of our stores for franchise will help us maintain and grow our presence on the high street.
"It will free up capital that we can invest in our business, enabling us to continue to grow and deliver for our people and customers."