Michel Roux Jr at his restaurant Le Gavroche. He has previously called for better treatment of chefs. Photograph: Rex

Michelin-starred TV chef Michel Roux Jr has been paying some kitchen staff at his Mayfair restaurant less than the minimum wage, the Guardian can reveal, while charging over £60 for one starter.

Earnings at Le Gavroche, named this month as London’s “top gastronomic experience”, have been as low as £5.50 an hour – well below the £7.20 “national living wage” introduced in April – according to information provided by chefs who have worked there. Their working days sometimes exceed 14 hours.

Several people who have been employed by the former MasterChef judge told the Guardian they routinely worked between 62 and 68 hours a week for about £375 before tax. The legal weekly minimum for anyone aged over 24 for such hours should be between £446 and £490.

It means chefs preparing a menu which includes starters such as the £62.80 lobster mousse with caviar and champagne sauce have been earning less per hour than McDonald’s workers cooking burgers.

Customers at Le Gavroche, where the prix fixe menu with wine costs £240 per person, have included the Queen, Adele and Paul McCartney.

Following a Guardian investigation, a spokeswoman for Le Gavroche admitted the restaurant had paid some staff less than the national living wage and said there would be immediate pay rises for affected workers and reductions in hours for all staff. She declined to say how many current staff had been underpaid, only indicating it was fewer than a dozen.

The restaurant could now face an HM Revenue and Customs investigation into underpayment of the minimum wage. HMRC has powers to order the payment of missing wages to underpaid employees and fine employers up to double the total underpayment.

“It is clear exploitation and inexcusable,” said one chef who spoke on condition of anonymity. “Other restaurants pay their staff at least minimum wage, even if it is a struggle for them to do so. If any restaurant can afford to pay their staff the legal minimum, it is Le Gavroche.”

Le Gavroche menu, highlighting the £62.80 lobster starter.

The chef provided evidence suggesting they had worked between 63 and 68 hours per week at an effective pay rate of about £5.50 per hour. Days begin as early as 7am and end as late as 11.30pm, with only about an hour off between lunch and dinner services and sometimes as little as 15 minutes to eat staff meals.

The restaurant made almost £250,000 in profit last year, according to abbreviated accounts filed at Companies House. Michel Roux Jr is the majority shareholder and has been running the restaurant since 1991 when he took over from his father, Albert Roux, who opened it in 1967 and helped train culinary stars including Marco Pierre White and Gordon Ramsay.

Michel Roux Jr quit MasterChef in 2014 following a row with the BBC over his commercial endorsement of a potato brand for which he has made TV adverts.

Three other chefs told the Guardian they worked similarly long hours on salaries of about £19,000 a year, which worked out at less than £6 an hour.

Le Gavroche declined to comment on the length of the hours the chefs told the Guardian they had worked or the effective pay rates. However, a spokeswoman said an internal review had found some staff were working “over what [Le Gavroche] was expecting them to work. It’s not massive amounts of hours beyond that, but enough that they reviewed it and they would need to give them some more money to make up the hours they were working.”

Chefs at work in Le Gavroche’s kitchen. Photograph: Le Gavroche PR

“I don’t agree with it,” said one chef. “I don’t like that they can get away with it, but I think because Roux has such a name he can do it. They are exploiting people.”

“Michel Roux and his team are supportive employers, but I also think the pay is disgusting,” said another.

The admission of underpayment will be embarrassing for Michel Roux Jr, who in February spoke out about staff workload when he announced he was closing the restaurant on Mondays.

“It is important that all high-profile chefs take a stance towards this – cutting back hours, better staff retention, better staff working lives – to make our industry more appealing,” he told Foodism, an industry magazine. “There is a shortage of quality chefs, so we need to treat our chefs and our staff better, and make it a better place to work.”

One chef who has worked at Le Gavroche said: “It angers me that he says one thing and does the other. You become a chef because it is your passion, so you take the low pay and the conditions because that is how it is, but after a while you begin to ask why … It needs to change for all of us. What does it say when minimum wage would be an improvement? I want to see him and the restaurant lead by example and do what needs to be done.”

According to official statistics, one in four workers in UK hotels and restaurants last year earned near or below the minimum wage, four times the level of the total workforce.

None of the chefs the Guardian spoke to said they had received any of the 13% service charge added to diners’ bills. Le Gavroche insisted all staff did share tips.

Michel Roux Jr. A spokeswoman for Le Gavroche said staff would get a pay rise and reduction in hours. Photograph: La Gavroche

Chefs’ terms and conditions state they “do not have any normal hours of work … You may be required to work additional hours … without further pay or time off in lieu.” They are also asked to sign a waiver exempting them from the legal 48-hour maximum working week.

In a statement, a spokeswoman for Le Gavroche said it had “recently undertaken one of its regular salary reviews, which revealed that a few staff were working longer hours than anticipated and the restaurant is increasing salaries for those staff with immediate effect to ensure that the entire staff is now earning the minimum wage or more for their age”.

“Michel Roux Jr has stated that it is his continued stance to reduce the number of hours that front of house, chefs and kitchen staff work,” the statement added. “To this end Le Gavroche announced earlier this year that the restaurant would be closed all day Monday, as well as being closed Saturday lunchtime, and all day Sunday. This was introduced to improve the working conditions for all staff members.”

By March 2017, Roux will “reduce the maximum estimated working hours for all to 50 hours a week”.

“Le Gavroche will implement this through closing the restaurant for lunch on Tuesdays, and taking last orders for dinner at 9.30 pm,” the statement said.

A job at Le Gavroche is considered valuable for young chefs, who get to learn fine French cooking skills under an acknowledged master and leave with a two-star Michelin restaurant on their CV. Many come from mainland Europe but frequently stay for only a year or two. The kitchen does not operate a shift system, rather all chefs work every service the restaurant is open. It ensures consistent quality but means the onus is on chefs to work however long it takes to get their job done, sources said.