Scotland’s Lord Advocate Dorothy Bain QC announced on Wednesday that those caught with Class A substances – such as heroin or cocaine – could receive a police warning rather than prosecution.
The scheme will allow police to use their discretion on cases of individual use only, while those caught supplying drugs to others will still face criminal charges.
In an ITV interview on Thursday, the Labour leader said Ms Bain’s decision was “probably the right thing to do”.
But Sir Keir said there was a “world of difference” between the reform and “ripping up” drugs laws entirely.
His comments come after Scottish Labour backed Ms Bain’s decision on Wednesday.
Under current laws in Scotland, officers can already use their discretion to issue warnings to those caught with Class B and C drugs – such as barbiturates and cannabis.
Following Ms Bain’s announcement, this has now been extended to Class A substances, which also include ecstasy, crack cocaine and magic mushrooms.
In the interview with ITV’s Representing Border, Sir Keir said: “There is a world of difference between a decision not to prosecute a particular case and ripping up the drug laws.
“It is not unusual in any legal system for those caught with small amounts of cannabis not to be prosecuted.
“I don’t think many people would argue that that discretion isn’t sensible.
“The very same in Scotland – there is a world of difference between that exercise and saying ‘do you think drug laws should be scrapped?’ to which my answer is no.”
However, when asked about Ms Bain’s decision, Sir Keir said: “It’s probably the right thing to do.
“It’s an independent decision that has been made.”
Describing the change as a “smart use of the law”, Scotland’s drug minister Angela Constance hailed the move as “very significant” as Scotland aims to reduce drugs deaths – which reached a record 1,339 in 2020.
— Priti Patel (@pritipatel) September 23, 2021
She added the change will only be an option in cases of possession for individual use, not where someone is suspected of being involved in supplying drugs to others.
Ms Constance told BBC Radio Scotland the change had been welcomed by all parties at Holyrood “with the exception of the Conservatives”.
Conservative justice spokesman Jamie Greene said the change means “people will now be receiving the same punishment for carrying Class A drugs as they would for urinating in public”.
He added: “Under the SNP, drug deaths have become our national shame, but their plans for an effective decriminalisation of drugs is not the way to solve this scandal.”
He insisted the Government “must rethink this dangerous approach, which dilutes how seriously we treat possession of the most deadly drugs in our society”.
Home Secretary Priti Patel hit out at Sir Keir following his ITV interview on Thursday, saying Labour is “weak on crime and weak on the causes of crime”.