By Martin Shankleman
Business correspondent, BBC News
Loch Fyne operates a chain of restaurants across the UK
The "ethical" fish restaurant group, Loch Fyne, pays staff at rates below the minimum wage, the BBC has learned.
It relies on customer tips to boost total pay to a lawful level.
Loch Fyne champions marine conservation, and says it is "an enterprise with respect for animals, people and ecology".
The Unite union called the company's behaviour "appalling", and said all restaurant staff should get the minimum wage, as well as a fair share of tips.
Staff at Loch Fyne Restaurants say they get paid £5.05 an hour, compared with the legal minimum wage of £5.52.
One waitress, who wanted to remain anonymous, said she asked about the pay rate at her interview.
"Jokingly, I asked, 'it's not the minimum wage is it?' They said 'well actually it is; eventually you get paid minimum wage, because it gets made up in tips'," she said.
On its website the company claims: "What sets us apart is our attention to detail and focus on quality. In everything we do, be it building a new restaurant, hiring staff or cooking the food we take huge pride in doing it to the best standard possible."
The waitress said "hardly any customers realise how their tips are used".
"Many customers shove tips on the (credit) card and think you are going to get it," she complained.
Restaurants are legally allowed to include tips in the calculation of employees earnings, but the practice has been criticised as unethical.
In a statement, Loch Fyne Restaurants said it observed the British Hospitality Association's guidelines, and "no-one is paid below the minimum wage".
It pointed out that the Loch Fyne Restaurants business was owned by the Greene King brewing group "so it follows its policies".
Unite is pressing the government to alter the rules on the minimum wage, to stop restaurants including tips in the calculation of employee earnings.
But experts have warned that the area is fraught, and any changes could lead to a legal challenge.
Loch Fyne Restaurants was set up in 1998, and employs 1,200 people at 44 sites across the UK.
Last week the BBC revealed that staff at the Hard Rock Cafe in London were paid less than half the minimum wage, with waiters on £2.06 an hour.