Fifteen years after the launch of the national minimum wage some employers still view paying it to their workers as a choice and offer elaborate excuses when caught out, according to HM Revenue & Customs, which has published a list of some of the worst.

One employer claimed that a worker was his wife but then had to ask her name. Another claimed that their employees didn't speak English, so were not entitled to the minimum wage, while others simply tried to deny an underpaid employee worked there at all, the Revenue said.

In the current tax year, employers must pay workers over 20 years of age at least £6.31 an hour, 18- to 20-year-olds £5.03 and 16- and 17-year-olds £3.72. Companies that fail to do so face fines and bills for backdated wages.

HMRC investigates employers who are reported as not paying the legal minimum. In 2013, these investigations resulted in more than 26,000 people getting a share of £4m in back pay.

The Revenue's list of employers' worst attempts to avoid being caught included:

• An employer said a woman on the premises was not entitled to NMW as she was his wife. When asked what his wife's name was the employer said: "Err … her name … err, what's your name love?"

• An employer told HMRC: "I don't think my workers know anything about the national minimum wage because they don't speak English."

• Another employer said: "When the national minimum wage goes up I do increase the amount I pay a little, even if the total pay is still below [it]. I don't think its right to ignore the rises."

• A number of employers paying rates below NMW, suggested that the accommodation they provided made up for the shortfall.

• An employer told HMRC: "It wasn't a conscious decision to say 'I'm not going to pay this', but I've never really considered doing it because I've not had people come to me and say, 'I'm not getting paid enough' or, 'Is this the minimum wage?'"

• An employee ran out of the premises when HMRC officers arrived to check for NMW infringements. The employee then returned – minus the work pinafore – pretending to be a customer.

Jennie Granger, director general of enforcement and compliance at HMRC, said: "Most employers are honest and pay their staff the correct rate. But this research shows that some still view the national minimum wage as a choice and will even try these crazy excuses to avoid paying workers what they are due."

HMRC said calls to its helpline from interns who are working for nothing, or for "expenses only", are being fast-tracked to enforcement officers for investigation.