Keir Starmer has backed a move to relax drug laws in Scotland for people found in possession of Class As such as heroin and cocaine.
Scotland's Lord Advocate Dorothy Bain QC announced that those caught with Class A substances could receive a police warning rather than prosecution.
Police will be able to use their discretion in individual cases - but those supplying hard drugs will still face criminal charges.
The Labour leader, a former Director of Public Prosecutions, said the decision was "probably the right thing to do" - but said drug laws should not be ripped up.
Officers can already use their discretion to issue warnings to those caught with Class B and C drugs - such as barbiturates and cannabis - under current Scottish laws.
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, Mr Starmer 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."
But when asked about Ms Bain's decision, he said: "It's probably the right thing to do.
"It's an independent decision that has been made."
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.
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 seized on Mr Starmer's words, saying Labour is "weak on crime and weak on the causes of crime".
