The union says that if ministers go ahead with plans to raise the earnings trigger, the earnings level from which employees will be automatically enrolled, for people to be auto-enrolled into workplace pensions, hundreds of thousands of low-paid workers will miss out unless they voluntarily sign up to the scheme.

Under pensions auto-enrolment, which started for employees in some of the UK’s biggest companies earlier this month, employers and employees will pay contributions on earnings above £5,564, but only if staff earn above £8,105.

However, the government is currently consulting on raising the lower earnings limit and the earnings trigger to £5,720 and £9,205 respectively.

Brendan Barber, TUC general secretary, said: “With two thirds of employees no longer saving into a workplace pension, auto-enrolment cannot come soon enough if we are to start tackling the UK’s growing pensioner poverty crisis.”

TUC analysis of official earnings data shows that around 3.5m workers earn less than £8,106 a year, and would therefore not be automatically enrolled into a workplace pension.

Raising the trigger to £9,205 next April will mean a further 585,000 staff will fall below the earnings trigger.

“It’s disappointing therefore to see that over half a million low-paid workers could soon be missing out. Women and part-time workers are the least likely to save into a pension.

“They should be a core target for auto-enrolment, not the main losers from government plans to restrict access to the scheme.”

Of those workers set to miss out on auto-enrolment if the earnings trigger is raised, 457,000, or 78%, are female and 506,000, or 86%, are part-time workers.

Low-paid women and part-time workers are the least likely to be saving into a workplace pension and should be a core target for auto-enrolment, not the focus of those excluded from it, says the TUC.

The union also points out that if the government continues to raise the earnings trigger in line with the personal allowance, which will eventually rise to £10,000, an additional half a million workers could miss out on auto-enrolment.

“The government should show they are fully committed to auto-enrolment by ensuring that as many people as possible are eligible to be enrolled into saving for a pension.”

The TUC is calling on the government to freeze the earnings trigger for auto-enrolment at £8,105, as well as the lower earnings limit at £5,564.

The TUC also wants the upper limit on employer contributions to be increased to £42,971 so that the proportion of earnings upon which pension contributions are made steadily rises each year.