Oliver Letwin has renewed his attack on Labour's "crypto-Kremlin" crime campaign.

The shadow home secretary warned on Thursday that the UK could lose its precious "innocent until proven guilty" principle under proposed criminal justice reforms backed by Tony Blair.

Letwin's latest civil libertarian sally, follows his claim on ePolitix yesterday that Labour's "big brother" approach could lose centre left support.

Speaking at a police conference, the Conservative home affairs frontbencher cautioned that plans to review "double jeopardy" - a principle that prevents people being repeatedly tried for the same crime - could damage the "presumption of innocence" protection at the heart of British justice.

"I accept the arguments for supposing that there are problems arising from the double jeopardy rule," he said.

"But I do not accept that that need lead us hastily into an overturning of that rule to the point where there are people being tried in what will be, unless there are very carefully constructed safeguards, the presumption of guilt."

In a move that will not play well to true blue Tory heartlands, or to Daily Mail leader writers, Letwin sounded a note of alarm over the erosion of defendants' rights. Rights, the government argues, need to be "rebalanced" in a bid to drive up conviction rates.

"Let us not, in designing a system more efficient, faster, and fairer for victims and witnesses, get to the point where we have a system that does not command public confidence because those who are innocent cannot start with a presumption of innocence," Letwin said.

The shadow home secretary repeated his intention of playing a parliamentary endgame in the Lords with controversial police reforms, and accused David Blunkett of trying to turn the Home Office into a "crypto-Kremlin".

"I hope the home secretary will withdraw and if he does not withdraw I shall seek through my colleagues in the Lords to compel him to withdraw those parts of the Police Reform Bill which seek to run every part of every police force in Britain from a desk in Whitehall,'' he said.