Racing NSW has asked senior politicians to consider declaring the Everest Cup race day a public holiday in New South Wales, similar to the Melbourne Cup.

Guardian Australia has confirmed that Racing NSW recently floated the proposal for a state holiday with the state opposition leader, Labor’s Luke Foley, as a way of further promoting the horse race as NSW’s signature event.

Racing NSW suggested moving the race, which has $13m in prize money, the largest in the world, to a Wednesday afternoon in October and declaring a half-day public holiday.

The proposal, similar to Victoria’s Melbourne Cup holiday, is certain to be controversial with employers and is set to further inflame the already heated public debate over the use of the Opera House sails to promote the barrier draw for the Everest Cup.

The Opera House CEO, Louise Herron, attempted to veto the Racing NSW proposal because it involved projecting logos on to the heritage-listed building, but was overruled by the premier, Gladys Berejiklian.

A spokesman for the premier’s office said they had not been approached about the public holiday.

But a spokesman for Foley confirmed the proposal had been floated with the opposition in recent weeks.

Foley is said to have been noncommittal and told the Racing NSW chief executive, Peter V’landys, that he would need to convince the state’s employers if he wanted to advance the idea.

However Labor has publicly backed the idea of making the race a “major event” which means it would be exempt from charges for services like policing.

The proposed half-day holiday would fall just two weeks after NSW Labor Day, which falls on the first Monday in October.

Racing NSW denied making a formal application for the public holiday, but did not directly respond to questions about discussions with Foley.

The chief operating officer for Racing NSW, Graeme Hinton, said: “There is no application from Racing NSW in relation to a proposal of that kind.”

The public holiday idea is said to have come after Racing NSW gave up on an earlier plan to install lights at Royal Randwick racecourse and run the Everest Cup in the evening.

The cost, which was estimated at $25m, was too high. Randwick racecourse is also in the middle of a built-up area and would pose significant noise, light and crowd issues.

But the proposal for night racing may still have legs.

Foley has said he would “work to install lights at Royal Randwick to grow the Spring Carnival and boost other races in the lead-up to the Everest.”

The NSW government’s decision on Friday night to order the Opera House to accept an application from Racing NSW came after a public campaign by the rightwing broadcaster Alan Jones and despite Opera House management saying the proposal contravened a policy to prevent the building’s commercialisation.

It has prompted an outcry amid concerns about the use of the building – a petition opposing the decision had almost reached more than 180,000 signatures on Monday afternoon.

Questions have also been raised about the legality of the promotion. On Monday afternoon the National Trust NSW conservation director, Graham Quint, said he believed projecting commercial material on to the sails contravened state laws.

“They’ve gone with a decision that we think probably is not legal,” Quint said. “It may be in breach of the Heritage Act.”

On Monday the NSW Greens called on the state’s Heritage Council to immediately issue a stop-work order to prevent the projection of the Everest barrier draw on the Opera House sails.

“This is a black-and-white issue,” the Greens MP David Shoebridge said. “The Opera House’s conservation management plan says explicitly it should not be used as a giant billboard.”

Clause 4.14 of the plan, required under the state’s heritage laws, says: “The Sydney Opera House exterior, particularly the shells (and even the Tarpeian Wall face), should not be regarded as a giant billboard or commercial / advertising opportunity. While the reality is that most events and celebrations will require commercial sponsorship, such sponsorship should not be on display in anything other than an appropriately discreet manner.”

On Monday the Labor frontbencher Anthony Albanese said the NSW government should support Herron’s push for a more “minimal” promotion of the race.

“The fact is that there’s agreement the colours of the horses should be used and the numbers should be used [but] this is a dispute over whether the trophy should be used,” he said.

“I don’t have a strong view frankly but given the Opera House board and Louise Herron have a view it would be reasonable their view be respected.”

Albanese has received criticism since he appeared to back the NSW government’s decision to order the Opera House to accept the application from Racing NSW.

Albanese on Friday told ABC radio people should “chill out a bit”, saying “people do associate Sydney with the Sydney Opera House”.

But speaking on Sky News on Monday, Albanese clarified his comments were made before Jones’ widely criticised interview of Herron – in which the controversial broadcaster berated the Opera House boss, saying “Who the hell do you think ... who do you think you are?”

“Alan Jones was rude, he was offensive and his behaviour was inexcusable,” Albanese said.

“[I think] that is one of the reasons why there has been such a strong response to this issue.”

Earlier on Monday Berejiklian stood by the decision, saying the government “would never support anything we thought diminished the iconic statute of the Opera House”.

“I am pleased the version we have landed on is a big compromise and it is different to what the government was presented with earlier on,” she said.