Diners across the UK will be able to enjoy half-price meals throughout August from Monday, as part of a government scheme aimed at boosting restaurants and pubs post-lockdown.
"Eat out to help out" applies to eat-in food and drink on Monday to Wednesdays at more than 72,000 venues.
The discount is capped at £10 per person and does not apply to alcohol.
But critics said unhealthy food should have been excluded from the scheme, over fears it could fuel obesity.
HM Revenue and Customs boss Jim Harra has also warned the scheme may not offer value for money for taxpayers.
The scheme is designed to encourage people to visit restaurants, cafes and pubs, which have been badly hit by the lockdown.
Around 80% of hospitality firms stopped trading in April, with 1.4 million workers furloughed - the highest of any sector - according to government data.
Many venues which have reopened since 4 July have also been forced to operate at a lower capacity to comply with social distancing rules.
The offer only applies at participating eateries in areas of the UK that are not in local lockdown - but major chains including Pizza Express, Costa Coffee, McDonald's and Nando's are among the 72,000 to have signed-up.
Chancellor Rishi Sunak said: "Our 'eat out to help out' scheme's number one aim is to help protect the jobs of 1.8 million chefs, waiters and restaurateurs by boosting demand and getting customers through the door.
"The industry is a vital ingredient to our economy and it's been hit hard by coronavirus, so enjoy summer safely by showing your favourite places your support - we'll pay half."
No vouchers are required to use the offer, with the participating establishment deducting 50% from the bill and charging the discount to the Treasury.
There is no minimum spend and you can use the discount as often as you like on eligible days. However, it does not apply to takeaways.
Officials said there had been more than 3.3 million hits on the "eat out to help out" restaurant finder website since it started up last week.
The start of the scheme comes only a week after Prime Minister Boris Johnson unveiled the government's new strategy to tackle obesity.
Liberal Democrat MP Munira Wilson, the party's health spokeswoman, said with research suggesting obesity increases someone's risk of dying from coronavirus, the chancellor should have prevented people using the discount to buy junk food.
"We all recognise the need to support the high street through the pandemic, but the government should have been more discerning with this scheme," she said.
"With a number of fast-food chains signing up to the scheme, it seems clear that public health did not factor into the government's decision.
"The government must put public health first and exclude from the scheme meals and drinks proven to contribute to obesity. We cannot afford to risk lives as we reopen the economy."
The scheme has also drawn criticism from some anti-obesity campaigners, with the National Obesity Forum saying it would be a "green light to promote junk food".