By James Landale
BBC News 24 Chief Political Correspondent
One of Lady Thatcher's closest allies is attacking David Cameron's decision to shift the Conservatives to the left.
Mr Cameron: Taking a huge risk?
Robin Harris, ex-speechwriter and member of Lady Thatcher's No 10 policy unit, will accuse Mr Cameron of a "potentially disastrous" strategy.
In the next edition of left-of-centre Prospect magazine, Mr Harris says: "He should be having sleepless nights."
He says it is a "huge risk" to assume that core Tory support will remain whatever policies are adopted.
Mr Harris, a consultant director of the right-of-centre think tank, Politeia, claims that the Cameron strategy is "treated with thinly disguised contempt by the policy experts, opinion formers and intellectuals clustered round many of the Tory think tanks".
Mr Harris will say that party donors "are unlikely to feel much sympathy" for the Cameron approach.
In the article he will say that Mr Cameron has "systematically repositioned the party to the left", so much so that he wants to change the Conservative Party "into an institution with no obvious purpose other than to ape New Labour".
Mr Harris will also accuse Mr Cameron of underestimating Gordon Brown and say he is wrong to assume that the chancellor will pursue more extreme socialist policies than Tony Blair.
Mr Harris will compare Mr Brown positively with Lady Thatcher - "Like Thatcher, Brown is immensely able, a workaholic, driven by values from a Protestant upbringing".
In contrast, although Mr Harris will describe Mr Cameron as "bright and unpersonable", he will add: "Up against Brown, he may look unprincipled and insubstantial".