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Britain sold chemicals and components to Syria that ended up being used in the manufacture of the deadly nerve agent sarin, BBC Newsnight can reveal.

A leaked Foreign Office document says they were supplied in the mid-1980s.

Foreign Secretary William Hague said UK firms provided the materials and that Syria has admitted they played a role in its chemical weapons programme.

He added there were no proper regulations at the time, but tighter rules and controls exist now.

In a written ministerial statement released on Wednesday, he said: "We judge it likely that these chemical exports by UK companies were subsequently used by Syria in their programmes to produce nerve agents, including sarin."

Sarin has been linked to a number of attacks in Syria's bloody three-year conflict.

'Tipping point'

A report by UN chemical weapons inspectors found "clear and convincing evidence" that surface-to-surface rockets containing sarin were fired at suburbs to the east and west of the Syrian capital Damascus last August - an attack that killed hundreds of people.

The US and UK accused President Bashar al-Assad's government of being behind the attacks, but Damascus blamed rebel groups fighting to topple him.

However, US, UK, French and Israeli officials have said there is also evidence that Syrian government forces used sarin against rebels and civilians on several previous occasions.

Image copyright AFP Image caption The conflict in Syria has created thousands of refugees

Attacks like those in Damascus proved the tipping point for the West - the redline that encouraged David Cameron and Barack Obama to consider military action to stop the conflict.

The UK took a leading role in forcing the Syrian regime to destroy its stockpiles of chemical weapons - and disclose to inspectors which countries were guilty of providing the materials.

Sarin building blocks

Syria's declarations to the Organisation for the Prohibition of Chemical Weapons (OPCW) name Britain as the sole supplier of dimethyl phosphate (DMP), trimethyl phosphate (TMP) and hexamine - three chemicals that Syria used in the production of sarin.

The phosphate chemicals are regarded as the building blocks of sarin and Newsnight understands British companies sold hundreds of metric tonnes of them to Damascus.

Syria's declarations have since been verified by a Whitehall audit of chemical sales by British companies.

What is sarin?

Image copyright AFP Image caption UN inspectors collected samples following allegations of chemical weapons strikes last year

Sarin is a neurotoxic organophosphorus compound that is highly toxic and potentially lethal

It is considered to be 20 times as deadly as cyanide

It is impossible to detect because it is a clear, colourless and tasteless liquid that has no odour in its purest form

It can also evaporate and spread through the air

Sarin causes the heart and other muscles - including those involved in breathing - to spasm

Sufficient exposure can lead to death via asphyxiation within minutes

Electrical fans that were thought to have only commercial uses were also sold to Syria in 2003, but they too ended up being used in the manufacture of sarin.

Ministers said that the chemicals were provided at a time when there were no proper regulations of such trade.

However, Britain must have been aware that something was amiss because, in 1985 and 1986, it rushed through new rules to stop certain chemicals being shipped to Syria over fears they were being weaponised.

Image copyright AP Image caption Chemical agents from Syria were last week taken to an Italian port ahead of their destruction at sea

A swathe of tighter rules and export licence controls now exist and the sales of TMP and DMP are heavily regulated.

Ministers believe it would be very difficult for such material to end up being sold in this way again.

The Foreign Office document makes clear Britain was not the only country guilty of selling materials to Damascus that ended up in its chemical weapons programme.

It states that "16 countries have been listed as suppliers to the Syrian programme. Of these 7, including the UK, provided both chemicals and equipment parts".

'Pinnacle of weapons'

But the document also makes clear this is not just an embarrassment from the distant past.

It explains: "Syria's declarations also list the UK as having supplied a number of electrical component parts, including fans. A licence for the export of fans was issued in 2003; it would seem these were diverted to the Syrian programme."

This was in the year Iraq was invaded over weapons of mass destruction that did not exist.

Image copyright Reuters Image caption The transfer has taken place under the media's watchful gaze

Professor Alastair Hay, a chemical weapons expert from the University of Leeds, said: "Sarin is what you would say is at the pinnacle of chemical weapons.

"It is very toxic. It is a chemical that is quite versatile.

"The amount that is required to kill somebody is very, very tiny and it can be dispersed very well and that is why a relatively small quantity killed in the region of a thousand people in Syria."

Referring to the period when the supplies were sold to Syria, he added: "I think at the time somebody should have been a bit more alert and saying... 'Why are we doing this? These chemicals have other uses but they can certainly be used in a chemical weapons programme and we shouldn't be selling them.'"