More than one million people spend 50 hours a week providing unpaid care, figures show.
One in ten people in England and Wales provide unpaid care, according to the Office for National Statistics (ONS).
Of the 5.8 million who look after family members and friends, 1.4 million people provide more than 50 hours a week of free care.
The figures from 2011 show that around 3.7 million provide between one and 19 hours of unpaid care a week and 775,000 provide between 20 and 49 hours.
The ONS said the number of unpaid carers has grown by 600,000 since 2001.
In Neath Port Talbot, south Wales, 14.6% of the population were unpaid carers - the highest proportion in England and Wales.
TUC general secretary Frances O'Grady said: "Britain's growing army of unpaid carers, the vast majority of whom are women, provide an invaluable service to society.
"But despite being worth around £340 billion a year, unpaid care is still not given the recognition it deserves from either Government or employers.
"Far too many women are forced to trade down or even quit jobs when they take on caring responsibilities. Not enough employers truly embrace flexible working, despite the benefits it holds for staff and the business.
"As the population ages, the ever-increasing care needs of our society cannot be met by unpaid care alone. We need Government investment in all services, from childcare to elderly care. Without this extra funding we will continue to see women drop out of the labour market to fill the gaps in care provision."