O'Grady will be speaking later today at a conference in Madrid, marking the 40 years of the European Trade Union Confederation (ETUC). She will tell the audience that if prime minister David Cameron "gets his way" over Europe, British workers - and possibly workers across the continent - will lose out.
O'Grady believes that if Cameron "gets his way", employees may no longer receive health and safety protection, equal treatments as part-time workers and women or paid holidays.
She will also tell the ETUC that Cameron has already made it easier for employers to sack people and more difficult for workers to get justice in the courts.
O'Grady will say: "What David Cameron is doing - if putting internal party management above the national and European interest wasn't bad enough - is even more sinister.
"The Government has already made it easier for employers to sack people they don't like and more difficult for workers to get justice before the courts. Now it is trying to abolish wage protection for farm workers, and stop people injured at work getting their rightful compensation.
"But there's one set of workers' rights David Cameron can't touch. Those are the rights provided for by social Europe - paid holidays, health and safety, equal treatment for part-time workers and women, protection when a business is sold off, and a voice at work.
"The Prime Minister wants to 'repatriate' those rights, and not because he thinks he can improve them. David Cameron wants to make it easier for bad employers to undercut good ones, drive down wages, and make people who already work some of the longest hours in Europe work even longer. To do that, he needs agreement from the rest of Europe.
"And when the UK Government calls on your Government to give him the chance to undermine British workers' rights, we want your Governments to say no. Not just out of solidarity with us, but in the interests of your own rights, your own wages, and your own jobs.
O'Grady added: "Together we must make the case for a worker's and citizen's Europe, not a banker's and financier's Europe. If the EU is only about fiscal austerity, open markets and privatisation, then ordinary Europeans will increasingly question its legitimacy - and rightly so.
"For a generation, Europe prospered by balancing the interests of business and those of workers. It's time to rediscover that bargain - and the sense of solidarity that underpins it."
Grant Shapps, the Tory chairman, said the Government was not ignoring the country's perilous economic position by developing a new policy on Europe. "Europe is a big central issue which has a major effect on most people's daily lives and should not be ignored," he told Dermot Murnaghan today on Sky News.
