The Public and Commercial Services (PCS) union has objected after London's National Gallery appointed a private security firm with no prior consultation to guard artworks at its upcoming exhibition, Rembrandt: The Late Works.

CIS Security is being paid approximately £500,000 for the task and is advertising three-month contracts at £10 per hour to cover the exhibition, which runs 15 October to 18 January 2015. According to the advert, guards are expected to answer queries and provide visitors with advice, guidance and information.

The union said it was concerned about the lack of consultation before CIS Security’s appointment. PCS general secretary Mark Serwotka accused the gallery of “using public money to usher in privatisation through the back door”.

PCS has written to National Gallery director Nicholas Penny outlining its concerns. The letter states: "We wonder whether the international lenders to the Rembrandt exhibition will agree to their loans on this basis.

"We hope that further confrontation and damage to the gallery's reputation can be avoided by withdrawing this proposal."

Serwotka added: "Private security companies, driven by the need to make profits, have absolutely no place looking after world-famous works of art, and these plans should be scrapped.”

The appointment of CIS Security comes after the gallery's announcement last month that around 400 of its 600 posts would be outsourced later this year, including all visitor, security and ticketing services.

PCS is planning to “vigorously oppose” any move towards privatisation, which it says could lead to a deterioration in pay and working conditions, and a "knowledge drain" of experienced staff.

The union has asked the culture, media and sport committee to investigate the gallery’s wider privatisation plans, while a petition against the move has attracted more than 1,400 signatures to date.

A National Gallery spokeswoman said the gallery did not wish to comment.

A news analysis on the privatisation of museum services will be published in the September issue of Museums Journal.