HM Revenue and Customs is being urged to launch an investigation into tax and pay at ride-hailing firm Uber.
The GMB union said HMRC should exercise its legal responsibility for enforcement of the minimum wage laws and collection of taxes.
The move comes after an Employment Tribunal determined last month that Uber drivers are employees rather than self-employed and should be given rights such as holiday pay as well as be entitled to the national minimum wage.
Maria Ludkin, GMB legal director, said: "We have formally requested today that HMRC investigate the dark maze of tax avoidance at Uber.
"The recent judgment shows that Uber drivers are workers, employed by Uber. The company can no longer try to pass the buck of tax responsibility to drivers by pretending they are self-employed.
"HMRC must act now to enforce the UK national living wage/national minimum wage laws for Uber drivers.
"HMRC should recover employers' national insurance contributions and employee PAYE tax and national insurance contributions from Uber for the drivers.
"HMRC has a legal duty to ensure that the 30,000 drivers are paid the living wage/national minimum wage for all hours Uber drivers are clocked on."