Burger King and Domino's Pizza have emerged as the latest employers now known to recruit nearly their entire workforce on zero-hour contracts. It brings the total number of zero-hour hirings in the private sector exposed by the Guardian to more than 180,000.

The burger chain employs all 20,000 workers in its restaurants on zero-hour contracts, while more than 90% of staff at Domino's, or more than 20,000 employees, are also on the controversial contracts.

The latest revelations put more pressure on the Office for National Statistics to revise its data on the number of employees on zero-hour contracts. The seven private sector companies exposed as zero-hour employers by the Guardian employ at least 75% of the 250,000 official ONS estimate already, indicating that the true number could be far higher for the private and public sectors combined. One estimate by the Chartered Institute of Personnel and Development puts the numbers of UK workers on the contracts at more than one million.

The Burger King and Domino's Pizza admissions come as the British Retail Consortium added its voice to the call on employers to act responsibly when using the contracts. Burger King, which has 1,400 restaurants across the UK, employs its entire non-management staff on the contracts, which leave workers with no guaranteed hours of work each week. The rotas are decided a week in advance but can be subject to changes at the last minute. Staff can also be sent home early if there is not enough business.

In 1999 the company was forced to pay £106,000 in compensation to staff employed on the contracts after staff were made to clock off and stand around in stores unpaid until business picked up.

A spokeswoman for Burger King said: "Burger King UK is a franchisee-only operated business. As such we do not directly own or operate any restaurants in the UK. Franchisees, many of whom operate multiple brands, apply the process and policy they find most suitable for their business. This choice is at the discretion of the franchisee. Burger King continues to ensure that staff welfare is a top priority."

Domino's Pizza, whose founder Rumi Verjee has just been given a peerage in the Queen's birthday honours list, employs around 90% of its 23,000 staff on the contracts despite some having worked for the company for several years, putting in shifts of up to 50 hours a week.

The 20,000 staff on zero-hour contracts are given a rota a week in advance. However, hours can still change with some workers sent home shortly after arriving, with no pay. In some cases when a takeaway outlet has been refurbished, staff can be left with no work at all while the overhaul is carried out.

A spokeswoman for Domino's Pizza said: "In the UK our stores are majority-owned and managed by franchisees that are granted a franchise agreement with Domino's. All of our franchisees comply with employment law and Domino's is a responsible employer and adheres fully to government legislation."

These are the latest fast food outlets to be exposed using the contracts, which are the subject of a government review, after the Guardian revealed that McDonald's employs 82,800 staff on zero-hour contracts, with Subway also revealed as widespread zero-hour employer.

By comparison, bakery chain Greggs said it did not use the contracts, with the majority on 12 to 15 hours a week guaranteed. Greggs chief executive Roger Whiteside said: "To have a flexible workforce to call on for a business is obviously very attractive but we have certain values with our business, especially with looking after our staff and helping in the community. The idea of having zero-hours contracts doesn't fit in with that at all.I can understand why it would be attractive but it's not something we do."

Other retailers, including Tesco, Asda, Sainsbury's, Morrisons, John Lewis, Marks and Spencer, Argos, Homebase, B&Q, Pret a Manger, Waitrose and Next, have all confirmed they do not use the contracts.

Helen Dickinson, director general of the British Retail Consortium, which represents the vast majority of high street retailers, also spoke out for the first time on the issue. She said: "The key issue for zero-hours contracts is that where they are used, they are used responsibly. Our members don't tend to have them but in the right circumstances, zero-hours contracts they can offer mutually beneficial flexibility and certainty that for example students away at University might require to return to work during the holidays. Parents can also benefit from the flexibility as they juggle family commitments and colleagues moving towards retirement might see benefits as they begin to reduce their hours gradually."