By Martin Shankleman
Business correspondent, BBC News
UK restaurants are accused of not being upfront about staff tips
A campaign to stop restaurants abusing the tips paid to staff has been launched by the UK's largest union.
Lack of transparency means that service charges may be diverted into the firm's takings without the customer knowing, Unite has claimed.
Some firms keep part of the service charge paid, the BBC has learned.
Others, including the Italian chain Carluccio's, expected customer tips to make pay up to the minimum wage. Carluccio's declined to comment.
A BBC investigation found Carluccio's paid some waiters £3.75 an hour, which is below the legal minimum .
HAVE YOUR SAY
I'll only tip if the service is beyond what I'm paying for
Dave, Barwell, Leicester
Restaurant groups were found to hold back a proportion of staff tips, including Pizza Express - which keeps 8% - and The Real Eating Company food chain in Sussex, which retains 22%.
Staff at one Tootsies restaurant in the home counties complained the company kept 60%. Tootsies declined to comment.
Helena Hudson of the Real Eating Company defended her company's tips policy, saying it reflected the administrative burden of distributing tips among staff.
"It's a service we provide, which I'd far rather not have to do," she said.
She denied staff were being ripped off.
"At least we don't use tips to top up the minimum wage," she added.
The Unite union is calling for changes to legislation to crack-down on those employers who use customers tips to pay their staff the legal minimum amount.
It also thinks restaurateurs should be more open with diners about their tips policy.
"We would like to see an obligation on employers to be fair and transparent about the way they distribute tips," Dave Turnbull from Unite told the BBC.
"All of this will be news to restaurant-goers who assume that all that they leave as a tip is going to the staff," he said.
Unite officials will protest later outside Pizza Express in Wimbledon, south London, against the sacking of a former employee whom they claim was fired for blowing the whistle on the company's tips policy.
Pizza Express said the man was sacked for mis-representing the company policy, by claiming it profited from the money it deducts from the tips.