A biographer seeking the release of the personal diaries of Lord and Lady Mountbatten, which had been saved for the nation using £2m of public money, says he has been spied on by the British state during his years-long legal battle with officialdom which has cost £450,000 in legal fees.

Andrew Lownie has now gained access to a host of documents relating to him that were held by the government. Among them, he has said, are papers indicating officials monitored his social media accounts and public appearances.

“The inference was that this information might be useful to smear me,” Lownie said. “My crime? I’m an historian who pushes back against the censoring of our history by the government and highlights its failures to adhere to public records acts and the Freedom of Information Act.”

Lownie obtained the documents about him by submitting a subject access request, which gives people the power to compel an organisation to reveal what personal information it holds on them. In Lownie’s case, this meant reams of correspondence that revealed the government’s attempts to keep tabs on him.

“The monitoring by the Cabinet Office and the Foreign Office includes my social media accounts, a flyer for a talk I gave at a private club, details of a lecture at a Cambridge alumni weekend, and a library talk with an internal heading by the Cabinet Office of ‘Not just any cook-along this week’,” he wrote for the campaign group Index on Censorship.

“The Cabinet Office eventually admitted that it held so much material on me collected over the past five years – it estimated it would take more than 650 hours to collect the information – that my requests needed to be broken down into six-monthly periods.

“What it released showed that my activities were brought to the attention of Alex Chisholm, the permanent secretary of the Cabinet Office … that my speaking engagements, newspaper articles and crowdfunding activities were monitored; and that information on other parts of my life was also collected.

“This included employment tribunal and defamation cases which I had successfully defended and which had nothing to do with my freedom of information requests or activities as an historian.”

A government spokesperson said: “There is no basis to these claims.”

Lownie, who wrote a biography of the Mountbattens in 2019, was placed under the official spotlight over his efforts to force the release of papers contained in the vast Broadlands archive – 4,500 boxes of official documents that contain Queen Victoria’s letters to Lord Palmerston, the diaries of the 7th Earl of Shaftesbury, and the Mountbattens’ papers. He said legal fees in the case have reached £450,000.

The archive was bought by the University of Southampton, with the help of a grant from the National Heritage Memorial fund, in 2010 and the intention was that the information would be made available to the public. Instead, the university blocked the release of the diaries and correspondence from the Mountbattens that covered historical events from the abdication of Edward VIII to the independence of India, after seeking advice from the Cabinet Office.

Mountbatten, who was killed in an IRA bombing in 1979, was great-uncle to King Charles III and an uncle to his father, Prince Philip.

Even after Lownie secured the release of most of the material, some has remained under wraps. Yet he has said he is undeterred. “It’s made me even more determined to carry on researching and writing controversial books even if it doesn’t win me friends in high places,” he said on Thursday.