The Tory peer – David Maclean when he was an MP – said he felt "deeply, deeply uneasy" about programmes that allow the security services to examine the internet activities of British citizens without the consent of parliament.
In an interview with the Guardian, Blencathra said that the public had a right to know their internet data might be "lifted" and shared with US intelligence services – and that MPs should either vote to approve the surveillance programmes or put a stop to them.
He also condemned the fact that his committee scrutinising the data communications bill – subsequently killed off by the Liberal Democrats – was never told about GCHQ's existing mass surveillance capabilities. A joint memo from MI5, MI6 and GCHQ made no mention of them, he added.
"Some people were very economical with the actuality. I think we would have regarded this as highly, highly relevant. I personally am annoyed we were not given this information," said Blencathra, who was an ally of Michael Howard and considered on the right of his party .
Before the draft data communications bill was shelved, Blencathra's committee said it was very concerned that its proposed surveillance powers were far too wide-ranging and recommended changes to limit its scope.
Blencathra's intervention came hours after sharp criticism of the intelligence agencies by another peer, the former director of public prosecutions Lord Macdonald, who condemned the way the new head of MI5 had dismissed calls for greater scrutiny. Last week Andrew Parker said this would affect the way MI5, GCHQ and MI6 did their work.