Judges on a tribunal are to decide whether the cost of protecting members of the royal family can for the first time be disclosed to the public. The Guardian will on Wednesday challenge the government’s refusal to tell the public how much of taxpayers’ money is spent on protecting King Charles and other members of the Windsor family. The bill for providing security for the family is traditionally kept secret. The total bill for the round-the-clock measures, such as armed bodyguards and patrols at their many homes, could run into the tens of millions each year. The secrecy prevents the public from knowing the true overall cost of funding the monarchy. The Guardian has reported extensively on a culture of wide-ranging official secrecy that surrounds the operation of the monarchy. Over two days, the judges on the freedom of information tribunal will hear an appeal by the Guardian, which is asking for the cost of providing security for the royal family between 2017 and 2020. The paper has stipulated that it is not asking how this total is spent on individual members of the Windsor family, nor the precise capabilities that the money is spent on. The Home Office opposes the disclosure of this overall total, arguing that it is legally entitled to withhold it because the release even of information aggregated over multiple years would pose an unacceptable threat to national security. The Home Office has said disclosure of the overall bill could encourage would-be attackers as it could help their research to locate flaws in security measures that are in place to protect the Windsors. Its stance has been backed by the information commissioner, John Edwards, the regulator who adjudicates freedom of information disputes. Edwards agreed with the Home Office, ruling that publishing the combined total over three years would “give some indication of the annual cost when simply divided by three. The resulting figure may be surprisingly low and, were this the case, then it could put those concerned at significantly more risk.” The Guardian believes that publishing the overall protection bill for the three years would not provide any useful information to a potential attacker, and that disclosure of the figure would enhance democratic debate about the cost of the monarchy. The government has always maintained that the bill for providing protection for the royal family must remain hidden for security reasons. The vacuum means that not even a rough figure for the total is known. There have been estimates that the bill could be in the region of £100m a year. The main source of taxpayers’ funding for the monarchy, known as the sovereign grant, is currently worth £86m. This total does not include the security bill. The secrecy stretches to files in the National Archives that date from the 1930s. The public are blocked from reading one file concerning the Duke of Windsor’s protection between 1935 and 1939 even though it had been open for 20 years. The Home Office has denied a Guardian request to see four files about the Windsors’ security arrangements in the 1980s. Who qualifies for publicly funded security generates controversy. Last year, Prince Andrew reportedly complained to the Home Office that his official bodyguards were being removed, even though he is no longer carrying out official duties.