"Redundancy floodgates" will open if the government fails to extend the furlough scheme, according to the Unite union.
With the government's Job Retention Scheme coming to an end on 31 October, the union is warning that many workers in the UK should expect a "miserable Christmas".
The Chancellor Rishi Sunak has repeatedly insisted that the government will not extend the furlough scheme, but claimed he is looking at "creative" ways to protect jobs.
Today (Wednesday) marks 45 days until the scheme ends, the same amount of notice employers must give to workers that they will be made redundant.
Without a "clear and urgent sign" from the government, Unite believes "employers facing short-term struggles will issue redundancy notices".
Earlier this week, the Institute for Employment Studies (IES) reported that the jobs crises is twice as bad as the last recession in 2009 with 380,000 planned redundancies between May and July. And it is thought the number could rise as high as 735,000 by the autumn.
In a letter to the Prime Minister, Unite general secretary Len McCluskey said an indicator of support from the government would
"put a floor under struggling employers who are working hard to stabilise in the face of immense challenges.
"With our competitor nations announcing the extension or modification of their jobs retention schemes, we ask that your government recognises the need for UK businesses and workers to receive similar support."
Some economists and business lobby groups have called for the government to introduce targeted support, rather than a blanket furlough scheme for all employers. Such a move would ensure those in the worst affected industries, like hospitality, manufacturing, aviation and aerospace, would get some support.
A Treasury spokesperson said: "The furlough scheme has done what it was designed to do - save jobs and help people back into employment."