His edict comes after a series of high-profile gaffes involving ministers, or their spin doctors, re-telling the same story about spending as if it were new.
Ministers will not be allowed to talk about "new" money unless it is actually new. Instead they must use the formula: "This is money announced in the Comprehensive Spending Review which had not previously been allocated."
The move comes after senior ministers were caught out announcing funding several times over.
Whitehall insiders say the Prime Minister and his official spokesman, Alastair Campbell, are no longer prepared to put up with the embarrassment - or the bad press - that the practice is causing.
One said: "There is a feeling that the spin has gone too far and people have started to get their fingers burned for allowing an impression to be left that money already announced is new money."
The Minister of Agriculture, Nick Brown, was recently rebuked by MPs for misleading farmers over an aid package.
Mr Brown had told embattled farmers that they would receive pounds 150m in direct aid. But it emerged later that only pounds 1m of that amount was "new" money. In fact pounds 89m was from deferred cattle passport charges, which farmers would not have to pay, and pounds 60m was a grant to hill farmers, first announced last year. The only new money was pounds 1m for marketing schemes.
Mr Brown was accused of giving farmers "false hope" as the beef crisis continued.
The Deputy Prime Minister, John Prescott, was also accused of re-announcing cash for transport policies when he said there would be an pounds 80bn 10-year transport programme - but neglected to add that much of the money was "old money" for projects that had already been announced, including the Channel Tunnel and part of Railtrack's own 10-year pounds 27bn investment programme.
The rest of the funding would come from public-private sector partnerships such as the profits from privatising the National Air Traffic Systems, windfalls from the working-place levies and congestion charging, and the ring-fenced proceeds of the fuel escalator.
The practice, which some MPs believe has been going on across government since Labour came to office, has infuriated the Opposition.
A Conservative Party spokesman said last night: "Time after time we have seen government ministers supposedly announcing programmes and spending commitments which have already been unveiled to the country many times before.
"In our survey of so-called government announcements we even found one education commitment that in one way or another had been trailed 16 times before.
"The Government should concentrate less on presenting their policies to the country and more on the substance of what they are seeking to do."Reuse content