Time spent travelling to and from first and last appointments by workers without a fixed office should be regarded as working time, the European Court of Justice has ruled.
This time has not previously been considered as work by many employers.
It means firms including those employing care workers, gas fitters and sales reps may be in breach of EU working time regulations.
BBC legal correspondent Clive Coleman said it could have a "huge effect".
"Employers may have to organise work schedules to ensure workers' first and last appointments are close to their homes," he added.
'Health and safety'
Chris Tutton, from the solicitors Irwin Mitchell, told the BBC: "Thousands of employers may now potentially be in breach of working time regulation rules in the UK."
The court said its judgement was about protecting the "health and safety" of workers as set out in the European Union's working time directive.
The directive is designed to protect workers from exploitation by employers, and it lays down regulations on matters such as how long employees work, how many breaks they have, and how much holiday they are entitled to.
One of its main goals is to ensure that no employee in the EU is obliged to work more than an average of 48 hours a week.
The ruling came about because of an ongoing legal case in Spain involving a company called Tyco, which installs security systems.
'Bear the burden'
The company shut its regional offices down in 2011, resulting in employees travelling varying distances before arriving at their first appointment.
The court ruling said: "The fact that the workers begin and finish the journeys at their homes stems directly from the decision of their employer to abolish the regional offices and not from the desire of the workers themselves.
"Requiring them to bear the burden of their employer's choice would be contrary to the objective of protecting the safety and health of workers pursued by the directive, which includes the necessity of guaranteeing workers a minimum rest period."
Meanwhile, employment law barrister Caspar Glyn agreed the court's decision could affect "millions of workers".
However, Mr Glyn also said there had been much speculation that this ruling could allow workers on the national minimum wage to claim more money for the time they spend getting to work.
But he said this would not be the case.
"The national minimum wage is a UK right, it is not a European right. There's no European right to a national minimum wage.
"The minimum wage regulations in the UK do not count as work travel from home or to any workplace," he said.