The average woman employee "effectively works for 47 days of the year for free," according to the Trades Union Congress.
Trade unionist Paul Nowak says he wants to see the "closest possible" economic relationship with the EU.
The call comes amid fears the bill would be watered down as some of its key backers left the cabinet.
The TUC has urged the government to "stay on course" in delivering its employment rights bill - a major shake-up of workers' rights.
TUC boss Paul Nowak says the chancellor should not take "anything off the table" ahead of November Budget.
He had a key role in setting up the Wales TUC, the umbrella body for Welsh unions, in the 1970s.
TUC general secretary Paul Nowak tells the BBC proposed 2.8% public sector pay rise is not credible.
The government needs to rethink green policies to avoid destroying working class communities, unions say.
In a speech to the TUC, the PM says the government will "not risk its mandate for economic stability".
