Deputy general secretary Tony Burke said the union was "disappointed" that MEPs had approved an amendment that meant agency workers would only be guaranteed pay on a par with permanent workers after six weeks in a company.
The exemption will operate for the first five years after the directive becomes law and only applies to the UK and Ireland.
The GPMU has argued that temps should get equal rights from day one, and Burke said the six-week "loophole" would allow "unscrupulous companies and agencies to continue to exploit agency workers". "Once again we will see the UK lagging behind the rest of Europe," he added.
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