A public inquiry into claims a former Westcountry MP sold information to communist spies will not be established.
Documents found by the BBC suggested Conservative MP and ex-minister Raymond Mawby, who represented Totnes from 1955 to 1983, was in the pay of the Czech Security Service.
Mr Mawby, who died in 1990, was said to have supplied spies with a floor plan and security arrangements of the Prime Minister's Commons office and other information at £100 a time.
Current Totnes Tory MP Sarah Wollaston condemned the "posthumous trial by media". But ministers have refused her request for a full investigation in the House of Commons.
In a letter to Dr Wollaston, Home Office Minister James Brokenshire wrote: "It is the long-standing policy of the Government that the Security and Intelligence Agencies neither to confirm nor deny the existence of investigations or records, for reasons of national security.
"The agencies frequently receive requests from members of the public, media and others for information relating to investigations and this principle can only be upheld when applied consistently. Consequently, I cannot reply to your request."
Dr Wollaston said former MPs with links across to Eastern Europe claim reports were often exaggerated.
She said: "Raymond Mawby is not here to defend himself and it is a shame that we will not have the opportunity to examine the full facts behind these very serious allegations."