The Agency Workers Directive states temporary workers will get the same pay and basic working conditions as permanent staff after being employed for 12 weeks.
Great Britain agreed to the directive on the condition it would be allowed to opt out of the European rules restricting working hours.
Brendan Barber, general secretary of the Trades Union Congress, said too many agency workers in the UK have faced unfair treatment and injustice.
"Now agency workers will now finally have a fair deal and be entitled to the same pay as permanent staff doing the same job," he added.
Britain’s opt out of the working hours regulations could cause problems as members of the European parliament may reject their request.
The issue would then have to go to conciliation with the European Council of Ministers and Great Britain would be under pressure to give way.
