The 36 per cent pay gap between part-time and full-time workers is the main barrier to closing the gender pay gap and tackling in-work poverty, the government has been told.
The warning comes as the Office for National Statistics published its latest annual pay statistics at the end of last week.
The pay gap between full-time and part-time workers has hardly fallen in 30 years, claims the Trades Union Congress (TUC).
There are currently over eight million part-time workers in the UK, nearly three-quarters of whom are female. However, the proportion of men working part-time today is at an all-time high, with the number of male part-time workers more than doubling in the last 20 years.
A large number of part-time workers live in poverty, with an average wage of just £8 per hour. Two fifths of part-time workers in the UK earn less than the living wage of £7.45 an hour, while two in five part-time workers in the capital earn less than the London living wage of £8.55.
The lack of high quality part-time work is illustrated by the fact that the five highest paid occupations - aircraft pilots, chief executives and directors of advertising and PR, marketing and sales, and telecommunications firms - are all are dominated by men and have a negligible number of part-time positions.
In contrast, four of the five worst paid occupations - waiters and waitresses, bar staff, catering assistants and launderers - are dominated by women and have more part-time jobs than full-time ones.
Recent research from the Resolution Foundation found that while the UK has a high proportion of female part-time workers - the third highest in the OECD - the quality of part-time work is low compared to other countries.
More high quality part-time work is the key to reducing in-work poverty and closing the gender pay gap, says the TUC.
The TUC believes that extending the right to request flexible working to everyone, announced recently by the government, should help more people to adjust their work patterns without having to change jobs. However, employers still need to do far more to support part-time work, like encouraging job shares at a senior level.
TUC General Secretary Designate Frances O'Grady said: "We hope that figures published later today show that the gender pay gap is narrowing. 'However, any small progress will be cold comfort to the millions of mainly female part-time workers who suffer a far larger pay penalty."
She continued: "Most women become part-time workers to balance work and caring responsibilities. This shouldn't mean also having to abandon their careers and accept poverty wages. It cannot be right that two in five part-time workers don't even earn the living wage."
"No healthy modern economy should have an enduring gender pay gap and growing in-work poverty. Unfortunately common sense solutions such as senior level job shares and flexible working are rarely available in the private sector, and are now under attack in the public sector. Unless we change the way we work we will never eliminate the pay gap or tackle poverty," said Ms O'Grady.