Fresh legal action is being taken on behalf of workers in the so-called gig economy, involving drivers at online giant Amazon.
The GMB union is taking a case against logistics firm UK Express, which is based in Birmingham but employs staff across the UK.
The GMB said it believes the drivers fall into the same category as those at taxi firm Uber, and should be classed as workers, not self-employed.
The case is the latest being brought by the GMB to tackle the growing trend of what it calls "bogus" self-employment and gig economy "exploitation".
Maria Ludkin, GMB legal director, said: "This is another case in a long line of legal battles around bogus self-employment.
"Employers might not like paying the minimum wage or giving their workers the protections they're entitled to in the workplace, but I'm afraid it's not optional.
"UK Express deliver for some of the world's largest companies, in this case Amazon.
"The drivers delivering for Amazon, like Uber drivers and delivery drivers for DX, cannot be classed as anything other than employed when you look at the law.
"We don't get to pick and choose which laws we adhere to and which we don't like the look of.
"This is a much wider issue than individual companies like UK Express or Uber.
"This is about employment in 21st century Britain."