Cabinet ministers have been instructed not to campaign in public on either side of the issue, ahead of a vote on the Terminally Ill Adults (End of Life) Bill on 29 November.
Dozens of Labour MPs are thought to be still undecided about the plans and Streeting’s intervention, first reported by The Times, external, could be especially influential because of his position as the health secretary.
It is also notable because Streeting voted for the legalisation of assisted dying the last time the Commons voted, in 2015 – meaning he has changed his mind.
Last month, he told the Financial Times he was “struggling” with the issue, saying he could “buy into the principle” of assisted dying but was “not sure as a country we have the right end-of-life care available to enable a real choice on assisted dying”.
He has also spoken of concerns about a “slippery slope” – which Archbishop of Canterbury Justin Welby has also warned of.
Streeting is the second cabinet minister in two days to state their intention to vote against changing the law, after Justice Secretary Shabana Mahmood told The Times of her “unshakeable belief in the sanctity and the value of human life” on Tuesday.
The pair head the two government departments, health and justice, that would be tasked with implementing any new law.
Leadbeater has said patients with serious illnesses are suffering “horrible painful deaths” and that “people deserve a choice”.
The precise detail of her proposed legislation – setting out the circumstances which could lead someone to be eligible for assisted dying – is not expected to be published until closer to the Commons debate at the end of next month.
However, Leadbeater has indicated her bill would restrict assisted dying to terminally ill patients, and insisted there is “absolutely no question of disabled people or those with mental illness who are not terminally ill being pressured to end their lives”.