At least 3,000 workers in Government departments and agencies are paid less than the living wage while more than 100 are on zero-hour contracts, figures have revealed.
The numbers, unearthed in a series of parliamentary questions, were revealed as Theresa May pledges to make protecting workers' rights a central theme of her premiership.
The Prime Minister, who last week told her party conference that the Conservatives are "truly the party of the workers", has announced a review of employment practices.
Labour MP Frank Field, who tabled the parliamentary questions, welcomed the review but urged Mrs May to practise what she preaches and begin by rooting out poor pay and insecure contracts in Government departments.
Mr Field, chairman of the Work and Pensions Select Committee, told the Press Association: "Mrs May has made the clearest declaration of a prime minister in many a day that she is aware there is a vulnerable, weak underbelly in British society and that she intends to defend them.
"And one really good way would be for her Government to ensure that all workers, whether paid directly or indirectly but in fact on Government work, reach a standard.
"Surely as part of your moral crusade for best standards you put into practice first what you are preaching to others?"
According to the figures, 1,272 members of staff employed by the Ministry of Defence and its agencies are paid less than the living wage as defined by the Living Wage Foundation.
It is currently set at £8.25 an hour in the UK and £9.40 an hour in London.
The minimum wage for over 25s, which was rebranded the national living wage by the Government, is lower at £7.20.
The figures also show that 913 workers at the Ministry of Justice and 490 at the National Offender Management Service are paid less than the living wage.
At the Department for Communities and Local Government (DCLG) 111 outsourced staff are paid below the living wage, while 28 workers are on zero-hours contracts.
In total, 3,061 workers, including in-house employees, outsourced and agency staff, were paid below the living wage across Government departments and agencies, while 113 were on zero-hours contracts, the figures show. Most of these were outsourced staff.
The figures emerged as public anger over low pay and insecure jobs has exploded following a series of scandals at Sports Direct and the delivery firm Hermes and the collapse of BHS.
Mrs May has pledged to act for the millions of Britons "just managing" by curbing company excess and has announced plans to put employees on company boards.
She has also appointed Matthew Taylor, Tony Blair's former policy chief, to carry out a review of employment practices.
Mr Field said public confidence in companies has sunk to a new low as many feel firms have lost their "moral map and compass".
He said that while some people may want to be on zero-hour contracts, others will have been pushed into them by their employers.
He welcomed Mrs May's pledge to improve the lot of workers, but warned that "she may have been making a commitment to protect a much larger army of vulnerable people than she actually thought".
A Government spokeswoman said: "The vast majority of Government employees earn above the threshold defined by the Living Wage Foundation.
"In a modern, flexible labour market zero-hours contracts may be suitable for a small proportion of the workforce where the contract suits their circumstances and work pattern.
"However, the Government's modern employment review, announced last week, will examine this in more depth and report in due course."