British labour laws unfairly prevent six million small business employees from joining trade unions, the Trades Union Congress (TUC) has claimed.
The TUC said the UK's trade union recognition laws were "irrational."
"Once again, UK workers find themselves getting a raw deal compared to workers in the rest of Europe," TUC general secretary Brendan Barber said.
Small business staff often need the most protection against low pay and poor safety measures, the TUC said.
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Under UK law, small and medium-sized enterprises (SMEs) with fewer than 22 employees do not have to recognise trade union affiliations among their workers.
The ceiling was lowered to 22 workers three years ago, extending the right union representation to thousands of workers.
"There is no logical reason to continue to deny the same rights to six million others just because they work for small employers," Mr Barber said.
But David Franklin, who runs a 20-man manufacturing company in Croydon, said small businesses were more likely to deal effectively with staff grievances than large organisations.
"If (the staff) have any problem, when you have 20 people, they can immediately talk to somebody," he told Radio Five Live.
"It is in our own interest to make sure that the staff we have are happy. I don't need anyone else to tell me this."