The UK risks fresh confrontation with Russia later when it asks for new powers for chemical weapons inspectors to assign blame for nerve agent attacks in Syria.
Inspectors can currently establish only whether such weapons have been used.
The UK foreign secretary will table the proposals, which are opposed by Moscow, at a special meeting of the chemical weapons watchdog (OPCW) at The Hague.
The meeting also comes in the wake of a nerve agent attack in Salisbury, Wilts.
Ahead of the OPCW meeting, which was been requested by the UK, Boris Johnson said the proposal was aimed at strengthening the ban on chemical weapons.
He tweeted: “We propose the @OPCW begins attributing responsibility for chemical weapons attacks in Syria. We also want action to support states to address the chemical terrorism threat.”
Mr Johnson said the OPCW was “the right body to study who is behind an attack”.
BBC diplomatic correspondent James Landale said British diplomats had launched a huge effort to win enough support to win a vote on the issue on Wednesday.
But Russia opposed the plan, which might see its Syrian allies face further international condemnation, he added.
Syrian opposition activists, rescue workers and medics say more than 40 people were killed in a suspected chemical attack on the Syrian city of Douma in April.
France said it had “proof” that “chemical weapons were used – at least chlorine – and that they were used by Bashar al-Assad’s regime”.
The Syrian government denied the allegation. And its key ally Russia said it had “irrefutable evidence” that the incident had been “staged” with the help of the UK.
Meanwhile, the UK says Russian ex-spy Sergei Skripal, 66, and his 33-year-old daughter, who were found slumped on a park bench in Salisbury on 4 March, were exposed to a nerve agent belonging to the Novichok group.
This analysis was backed by the international chemical weapons watchdog.
The attempted murder of the pair on British soil led to accusations of Russian state involvement. Russia has denied involvement in the attack.
Yulia Skripal left hospital a month after the attack, and her father was discharged a month later.
Alastair Hay, an expert on chemical weapons at Leeds University, said the UK was “absolutely right” to press for new powers.
“Whether it happens and whether there are sufficient votes that the UK can garner to support the move is still an open question really,” he told the BBC.
“Of course you have the UK, the Unites States, France and many others who want the OPCW to have those powers.
“But there are others who are very concerned about it. And that includes Russia and some of its allies. So, it’s divided and it’s a question of how many allies the UK can muster.”