The Home Office has prompted outrage by blocking the appointment of the man tasked with leading investigations into the UK’s spy agencies.

Eric King had been appointed head of investigations at the new government watchdog charged with regulating the intelligence agencies until the Home Office intervened, apparently due to his “previous work and associations”.

King is the former head of a coalition of groups campaigning for reform in the wake of revelations by US whistleblower Edward Snowden which also exposed a massive state surveillance structure in the UK.

His lobbying work on mass surveillance is believed to have helped him secure the key role at the government’s new Investigatory Powers Commissioner’s Office (IPCO).

King would have become the first head of investigation at the IPCO, whose brief is to “provide independent oversight of the use of investigatory powers” by the government’s monitoring agency GCHQ, the security services MI5 and MI6, the National Crime Agency and all police forces as well as other government departments.

King is a former director of the Don’t Spy On Us coalition, which campaigned for reform following the 2013 revelations by Snowden that exposed the massive capacity of Britain and America’s intelligence agencies to capture communications.

He was also deputy director for five years at Privacy International, where he worked on issues related to signals intelligence and human rights.

The official Home Office explanation that King says he was given was turned down on “national security grounds”, but he has said a subsequent conversation implied “it was a lot to do with my previous work and associations”. In a series of messages on Twitter, King, who tweets as Eric Kind under the handle @3i5, added: “It seems I have been refused clearance for a role holding the security and intelligence agencies to account, because of previous work holding those same agencies to account.”

King’s links to whistleblowers such as Edward Snowden probably cost him his new job. Photograph: Alan Rusbridger/The Guardian

He added in another post: “My best guess is that – in short – the vetting team has judged that because I have criticised the intelligence agencies and defended whistleblowers like Edward Snowden, I shouldn’t be entrusted with national security sensitive information.”

News that King will not be joining the 70-strong staff at IPCO has outraged campaigners who viewed his appointment as a sign that government wanted to open itself to scrutiny and prompted hopes he would be given sufficient access to secret information. Corey Stoughton, advocacy director of human rights group Liberty, said: “Eric was clearly recruited by IPCO precisely because of the invaluable experience he has in holding security services to account. It is hugely disappointing, if not surprising, that this experience makes him unsuitable in the view of the Home Office.

“Independent scrutiny of the security services’ vast power to spy on people is vital to IPCO’s credibility, and to ensuring the UK’s surveillance regime respects our right to privacy. This perverse decision highlights deep flaws in a system that enables the security services to veto the decisions of their own watchdog. It’s an unsustainable position. ”

King, a visiting lecturer at Queen Mary University London, where he teaches criminal justice and surveillance technologies, would have overseen internal inquiries into the interception of phone calls, the handling of MI5 and MI6 agents, and whether surveillance and powers permitting bulk collection of communications data were being carried out within the law.

King would have worked alongside 70 staff including inspectors, technical and legal advisers, as well as 15 serving and retired judges, with IPCO also granted the power of judicial checks.

His role would have supported the UK’s first investigatory powers commissioner, Lord Justice Fulford, a former judge at the international criminal court in The Hague.

King declined to speak beyond his social media messages but he has vented his frustration on Twitter.

He wrote: “I was told verbally by the vetting team that there was nothing they felt I had withheld or misled them about. There were no problems in my personal life, with finances, or other complications. Just my previous work and associations.

Another read: “The problem, at its heart, is that there’s a conflict as to whether my previous work and views are a positive or negative thing. They are both the reason I was hired and the reason my clearance was refused by the Home Office vetting team.”

A Home Office spokesperson said: “We do not comment on individuals’ security clearance.”

ICPO, which began work in 2017, combines the work of the previously separate surveillance commissioner, interception of communications commissioner and intelligence services commissioner.