BLAME for more than 100,000 Scots earning less than the national minimum wage “sits squarely at the UK Government’s door”, David Cameron has been told.

In a blistering letter, the Prime Minister is told he “cannot pass the buck” on the question of why so many workers are paid less than the legal limit and urged to “get on with it”. The call comes from Glasgow Central MP Alison Thewliss, who last week blasted the Department for Work and Pensions on “obscene” benefits sanctions for those who have found work. Ramping up her defence of workers, Thewliss has now challenged Cameron to go after “unscrupulous” employers who pay staff poverty wages.

The call comes after the Government’s own figures show vast numbers of workers in Scotland have hourly earnings below specified rates. As many as 130,000 jobs for over-21s are paid at less than £6.50, while 18,000 posts for those aged 18-20 earn under £5.30 per hour and 7,000 workers under the age of 18 are on an hourly rate of less than £3.87.

The voluntary Living Wage was set at £7.65 in 2013 but two years on, 112,000 Scots aged between 16 and 21 (68.6 per cent) still do not earn this hourly rate.

The figures, released last month, date from April – three months after the former Tory-LibDem Government led by Cameron pledged to crackdown on firms that exploit staff with illegally low wages.

Yesterday in a letter to Cameron, Thewliss said: “These figures are utterly appalling and I believe they pose a number of very serious questions for your Government to answer.

“Food bank usage in the UK has exploded in recent years and studies consistently show that a large factor in this proliferation of food poverty relates to low pay which is almost certainly backed up by these shocking figures.

“It is imperative that your Government acts without delay to go after unscrupulous employers who are still not paying their staff the national minimum wage – let alone a living wage, which is where we should actually be in 2015.

“Given that your Government has consistently failed to heed calls to devolve minimum wage legislation to Scotland, you cannot pass the buck on this issue.

“The blame sits squarely at the UK Government’s door. You need to get on with it. And if you can’t, then I’m sure the Scottish Government will.”

When it was first introduced in April 1999, the rate of the national minimum wage was £3.60 per hour, or £3 for 18 to 21-year-olds.

At that time, 1,900,000 people across the UK were believed to be paid less than that. Today the overall total of over-21s paid less than their statutory right is 1,718,000 – an improvement of fewer than 200,000.

However, even this level is lower than that needed for many workers to provide for themselves and their families. And while the government plans to bring in a new National Living Wage of £7.20 per hour from April for those aged 25 and over, campaigners say this is also an underpayment and are calling on more companies to adopt a voluntary rate of £8.25 for all staff over the age of 18.

There are currently more than 430 accredited Living Wage employers in Scotland. In September, UK Business Secretary Sajid Javid said there is “no excuse for employers flouting minimum wage rules” to “cheat” staff, announcing a stronger compliance regime and harsher penalties.

Yesterday the Department for Business, Innovation and Skills insisted it was standing up for workers, with affected staff entitled to up to six years of back pay and fines for underpaying workers due to double to 200 per cent of the sum withheld.

Carla McCormack, policy and parliamentary officer at The Poverty Alliance, said: “The number of people earning below the National Minimum Wage in Scotland is at an unacceptable level.

“The minimum wage is supposed to be a floor below which people should not fall, but these figures clearly illustrate that this is not the case.

“We believe that people should be paid the Living Wage, which is £8.25 per hour, and so to see businesses failing to meet even the minimum wage is incredibly disappointing.”

Meanwhile, Thewliss said in-work poverty is “one of the clearest signs that we are not all in this together and people aren’t benefiting from the UK continuing to have decision-making powers over pay”.

She added: “Serious questions need to be asked about how, 18 years on from minimum wage legislation being introduced, so many people are still being exploited and let down by this Government. One of the things these figures reveal is that young people lose out big time. The SNP is clear – we believe pay shouldn’t be based on your age. That is discriminatory and we will resist any moves, particularly with the new con trick that is the UK Government’s Living Wage, to further penalise young people.”