The UK Foreign Office is holding a conference to explain how it will finally place into the public domain millions of public records that it has unlawfully held for decades – but is refusing to allow members of the public to attend.

Selected historians and archivists have been invited to the event on 9 May, known as Records Day, but the FCO has said it will not admit the public or media.

Meanwhile, a basic inventory of the 1.2m files that have been posted on a government website has been altered, with all references to the cold war spies Guy Burgess and Donald Maclean having disappeared. An earlier version of the inventory made clear that the withheld files on the two men took up more than 4 metres of shelving.

The FCO has denied that references to the spies, who passed thousands of confidential documents to the Soviet Union during the cold war, had been removed to deter requests under the Freedom of Information Act.

The FCO is trying to find a way to transfer at least 600,000 files containing millions of papers that the department withheld from the National Archive at Kew in breach of the Public Records Act's 30-year rule. The files are held at Hanslope Park, a high-security compound in Buckinghamshire that the FCO shares with MI5 and MI6.

In 2011, the FCO admitted it had withheld 1,500 files about colonial Kenya from the high court during a court case brought by a group of elderly Kenyans who claimed damages for the mistreatment they suffered while imprisoned during the 1950 Mau Mau insurgency. Initially, the department denied 1,500 files existed, but changed its mind when confronted with evidence in court.

After the papers were handed over, the government expressed "sincere regret" and paid £13.9m in compensation to more than 5,000 prison survivors. It then said there were 8,800 colonial era files at Hanslope Park. It later emerged there were as many as 20,000 colonial-era files.

There are thought to be more than 500,000 other files at Hanslope Park that the FCO will not release, although some date back to 1852. Historians and campaigners for greater transparency have said the way in which the FCO has kept the files secret is a scandal. A number, such as Richard Drayton, Rhodes professor of imperial history at King's College London, have said that reform of the Public Records Act is overdue.

Tony Badger, a history professor at the University of Cambridge who is overseeing the transfer of the files to the National Archives at Kew on behalf of the FCO, said that while some of the files contained papers of little or no value, others were "extraordinarily valuable" and should be made publicly available as soon as possible.

The director of the Campaign for Freedom of Information, Maurice Frankel, said: "People will understand that the Foreign Office can't fix this problem overnight. But they have got to do this openly. They may not be able to get themselves out of this mess without a very large amount of work, but that doesn't mean that they should be solely responsible for choosing what to prioritise for transfer to the National Archives. There has to be involvement of people who may in practice try to make use of these files, and that includes members of the public."

The FCO has never fully explained how such an enormous collection of files came to be hidden from view, leading to suspicions that it was attempting to conceal material that could damage diplomatic relations, fuel litigation or that was simply embarrassing.

FCO minister David Lidington told MPs his department was meeting its legal obligations over the handling of public records with "maximum transparency". However, he has said he will not answer questions about the affair from the media. "The proposed records day is not a media briefing, therefore journalists will not be invited," a spokesman from the FCO said when the Guardian asked if it could attend the conference.

The FCO invited historians to another record day in May last year, to discuss the colonial era papers. Again, members of the public were not admitted. The department's record-keepers used the occasion to criticise media reporting of the affair, but failed to disclose that they were holding a further 1.2m files at Hanslope Park.