MPs looking into phone hacking at the News of the World were put under covert surveillance in 2009, it was claimed today.
Media commentator Roy Greenslade said the newspaper had used private investigators and journalists to trail "every member" of the Culture Select Committee.
The surveillance was ordered by a "senior executive" and continued for between three and 10 days, he told Sky News.
Mr Greenslade added that there was then a "complete change of heart at the top level" and "everyone was told to stop doing it, not to talk about and give up on the whole enterprise".
The current version of the committee is expected to discuss the claims when it meets tomorrow morning.
Labour MP Tom Watson cancelled his appearance at the Society of Editors' annual conference to seek advice from Speaker John Bercow and consider the "legal and constitutional" implications.
Louise Mensch, a Tory member of the committee, stressed there was "absolutely no proof" the claims were true but MPs would look into it.
"It is chilling, it is terrifying, it sort of says 'don't mess with us or we will come to get you'," she said.