Leading historians are calling on the UK's Foreign Office to "come clean" over its plans for a massive archive of public documents, which it has unlawfully kept hidden for decades, prompting accusations that it has been attempting to manipulate impressions of Britain's past.

The Foreign and Commonwealth Office (FCO) has hoarded 1.2m files – some of them dating back to the 1840s – in breach of the 30-year rule of the Public Records Act, which should have seen them transferred to the National Archive.

Such is the level of concern among some historians that a number of leading figures from Oxford, Cambridge and London universities are known to have discussed whether legal action may be necessary to secure the archive and to bring it into the public domain.

Some are concerned that major works about contemporary British and imperial history may need to be rewritten, while others believe that what they describe as a scandalous act of concealment underlines the need for a major overhaul of the system for declassification of government papers as public records....