David Cameron will today pledge £100m for a new drug rehabilitation programme in Scotland, claiming it to be the most significant step ever taken in Scotland to combat drugs.

Speaking in Aberdeen this morning, the Conservative leader will also scotch any prospect of his party going into the Scottish parliamentary election campaign with a pledge to cut income tax.

Annabel Goldie, the leader of the Scottish Tories, has fought vigorously to prioritise the fight against drugs in Scotland and has campaigned internally for any money to be spent on that fight rather than cutting the so-called tartan tax.

While the decision may disappoint traditional Conservatives, who have argued for a tax-cutting bonanza under a Tory regime, Mr Cameron fully supported the decision to leave Holyrood's income tax powers alone. It has promised a council tax cut for pensioners and has yet to say what it proposes on business rates.

The Conservative leader - and his entire Shadow Cabinet - are in Scotland today, preparing their Scottish parliamentary election campaign.

Speaking to The Herald in London before he headed north, he said: "I am going to Aberdeen specifically to visit a drug project and to work with the Scottish Conservatives who are bravely, sensibly and rightly saying that they are going to put a £100m into the fight against drugs and drug rehabilitation rather than offer tax cuts at the Scottish parliamentary election.

"I think that is absolutely right. It is their decision, and I think it is a very good decision. I think the agenda of tackling drug abuse is hugely important. So I am going to go and see the efforts first hand."

Mr Cameron's aides revealed that he had been shocked at the extent of the drugs problems in Scotland, with 37 new patients seeking treatment every day.

Aberdeen, where Mr Cameron will make his pledge today, has, like Scotland's other large cities, a serious drug problem.

Mr Cameron insisted party members should do what they thought was right rather than pursue a tax-cutting agenda to out-manoeuvre the other parties, two of which are considering cutting business tax.

While the Scottish elections may have given him a chance to demonstrate his tax cutting inclinations, he said: "It is a matter for Scottish Conservatives. We live in a devolved system and you have to believe in a devolved system head and heart, practice and theory. It is up to the Scottish Conservatives to come forward with their policy proposals.

"I think they have made a good call by saying they are going to spend money on drug rehab rather than tax cuts."

It is also possible that had there been a party clamour for tax cuts, Mr Cameron and George Osborne, the Shadow Chancellor, would have vetoed them for fear that they would hand Gordon Brown a stick with which to beat the Tories.

They desperately want to avoid their opponents presenting them as tax cutters before responsible guardians of the public services.