Drug users caught in possession of enough cannabis for just 10 joints could be classified as dealers and face up to 14 years in jail under proposals being considered in Whitehall.

The Home Office is looking at new thresholds on the amount of illegal substances classified as merely possession rather than intent to supply.

There was an outcry last year when the Home Office proposed that cannabis smokers should be allowed to have enough of the drug to make more than 500 joints and still claim it was for personal use. This appeared to reinforce the more "softly, softly" approach to cannabis possession that was behind the decision to re-categorise the drug from a Class B to a Class C drug.

But the Home Office has now performed what one MP yesterday called a "dramatic flip-flop" and is proposing far smaller thresholds which campaigners said could end up sending casual users to prison as dealers.

The Home Office has written to the Government's experts, the Advisory Committee on the Misuse of Drugs, telling them that ministers are minded to set the threshold for cannabis possession at just 5g - enough to make between 10 and 20 joints - compared with the 500g first proposed.

For ecstasy the limit would be five pills against 10 in the earlier plans while the limit for amphetamines would be 14g in line with the original proposal. Possession of cocaine, heroin or crack cocaine would be just two grammes against the seven proposed.

The documents also showed that the Government's official advisors on drug policy, the Advisory Council on the Misuse of Drugs, favoured a higher 10g limit for cannabis possession.

Nick Clegg, the Liberal Democrat home affairs spokesman, said: "This dramatic shift in policy shows the Government is in a state of total panic and chaos."

He added: "Labour's flip-flopping is simply not an adequate response."

David Davis, the shadow home secretary, said: "This is a move in a sensible direction, but continuous changes by the Government have only added to the confusion."