The Court of Appeal has rejected an appeal by Pimlico Plumbers on the employment status of a former worker.
Gary Smith, who worked for the London-based company for six years until 2011, had already won an employment tribunal case after challenging the company's view that he was self-employed.
His solicitor Jacqueline McGuigan said today's court decision could affect other workers in the so-called gig economy, such as delivery drivers.
"We are absolutely delighted. The decision brings welcome clarity to the issue of employment status relating to work in parts of the economy," she told the Press Association.
Mr Smith worked for Pimlico Plumbers as a plumber for six years until 2011, when he claims he was dismissed following a heart attack.
He took a case to a tribunal, which found he was a "worker" and so was entitled to employment rights.
The case focused on whether individuals engaged as self-employed contractors should be classed as workers.
Lawyers believe the decision will have a significant impact on the gig economy, which has seen a huge increase in the number of people working under self-employed contracts.
Pimlico Plumbers boss Charlie Mullins said outside the court: "I am happy. This gives some clarity.
"We will be looking at the full judgment and there is a good chance we will appeal to the Supreme Court."
Maria Ludkin, the GMB union's legal director, said: "This case, like the Uber case last October, is yet another victory for the bogus self-employed who have been treated appallingly by their employer.
"All they want is basic employment rights as are enjoyed by the majority, including the right to be paid a minimum wage and holiday pay.
"While these plumbers are making Charlie Mullins even more millions, he needs to get his house in order and start respecting basic workers' rights."
The GMB is supporting a group of Deliveroo drivers in Brighton who have given two weeks' notice to the company to offer better pay and more hours or face protests and strike action.
They are currently classed as "independent contractors", not workers or employees, and are not entitled to basic employment rights such as minimum wage, holiday or sick pay, said the GMB.
Southern regional official Paul Maloney said: "We stand with the riders against Deliveroo, another company trying to duck its obligations and responsibilities by making its workforce 'independent contractors'.
"All they want is basic employment rights as are enjoyed by the majority, including the right to be paid a minimum wage, the right to sick pay and holiday pay."