The National Gallery’s security staff, of whom the vast majority are from minority ethnic backgrounds, are facing shocking segregation, a senior curator has claimed.

Its outsourced guards are barred from the building’s high-quality “hub” – bright and airy recreation rooms that are available to the institution’s mainly white employees. They are not allowed to take their breaks there, or even sit in that area.

Instead, they are relegated to a “stifling” basement space with broken furniture, continuing problems with rodents and no natural light or air-conditioning.

Staff liken it to Upstairs, Downstairs, the television drama series that explored class relations in Edwardian England.

The curator, who wished to remain anonymous, told the Guardian: “It’s almost become a running joke within the National Gallery. Security staff are being treated like second-class citizens.”

The insider said gallery employees – everyone from curators to marketing staff – used the hub, which was created in previously underused spaces between and beneath the gallery’s exhibition halls, while outsourced security staff were prevented from doing so.

They said: “There’s this beautiful hub area. You’ve got all the in-house, largely white staff in there. This has got potentially divisive for the gallery in the sense that the poor facilities in a basement area are used by black and Asian security staff.”

While the hub has a heavily subsidised canteen and its own chef, the basement space is described as “dingy, dirty and dark”, with a kiosk offering little more than a KitKat and no cooked food.

“It wasn’t meant for people,” the curator said. “It was meant for storage.”

The National Gallery has reportedly told them that restricting the hub access is about avoiding overcrowding, but the security staff are still barred during weekends and holidays, when there are virtually no National Gallery employees using it.

The curator added that on any one day there were perhaps 60 guards on site, but barely a handful took their break at the same time. “So the argument that the hub’s overcrowded is probably pushing it a bit,” the curator said.

Security staff are represented by the PCS union, which believes they are being treated with “absolute contempt”, warning of the basement space: “Most dangerously, it is unventilated and accessible only by ducking under pipes.”

Security staff are also said to feel the gallery is putting the collection at risk in expecting an individual guard to cover as many as four rooms at once.

One observer said guards could only be in one room at a time and it was easy to distract them. Incidents in recent years have included activists from Just Stop Oil hurling tomato soup over Van Gogh’s Sunflowers and attacking Velázquez’s Rokeby Venus with hammers.

On 1 July, PCS wrote to the gallery’s director, arguing that forcing a largely black and Asian workforce into the basement while the largely white staff “enjoy fresh air, sunlight and decent break and food facilities” was “potentially discriminatory under the Equality Act 2010”. PCS is still awaiting a reply.

Nick Marro, the PCS culture group secretary, told the Guardian: “There is no excuse for the two-tier workforce … Our members deserve equal treatment.”

Outsourced security staff are employed by CIS Security, which declined to comment.

The National Gallery said: “Security staff do not have access to the hub offices. This has been the position since those offices were established due to wider space constraints, with dedicated facilities provided elsewhere in the gallery for security staff and other contractors.”

The gallery said CIS had recently conducted a staff survey on workplace facilities and was implementing improvements in response to the feedback received.

It said it had consulted security staff on what catering they wanted and they said they preferred to have a subsidised vending machine, which had since been installed.

“The gallery closely monitors its estate and works with specialist contractors to manage and address any pest-control issues as part of its routine facilities management programme,” it said.

“We do not comment on security operations, staffing levels or deployment arrangements, as these are kept under constant review and discussing them publicly could compromise security.”