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Chemical weapons experts investigating the attempted murder of a Russian former spy and his daughter will arrive in the UK tomorrow.

Inspectors will officially "kick off" a probe into Novichok, the nerve agent used in Salisbury, Wiltshire, on March 4.

Two weeks after being poisoned by the nerve agent, Russian ex-spy Sergei Skripal , 66, and his daughter Yulia, 33, continue to fight for their lives in hospital.

Now, independent analysts from the Organisation for the Prohibition of Chemical Weapons (OPCW), from The Hague, Netherlands, will meet with officials from the Defence Science and Technology Laboratory and the police to discuss the process for collecting samples.

These will then be dispatched to "highly reputable international laboratories selected by the OPCW" for testing, with results expected to take a minimum of two weeks.

(Image: Reuters)

This is the next step in the process to independently verify the analysis carried out by the Defence Science and Technology Laboratory at Porton Down .

Last Wednesday, Prime Minister Theresa May wrote to the OPCW to formally invite them to verify the Government’s analysis of the nerve agent used in the Salisbury attack.

Subsequently, the UK’s Permanent Representative to the OPCW wrote to the Technical Secretariat inviting them to come to the UK to take a sample.

On March 12, UK Foreign Secretary Boris Johnson summoned the Russian Ambassador and sought an explanation from the Russian Government.

"We received no meaningful response," a spokesman for the FCO said. "It is therefore Russia which is failing to comply with the provisions of the convention. We should resist any Russian attempts to muddy the waters."

(Image: PA)

Johnson revealed this morning the UK has information indicating that within the last decade, Russia has investigated ways of delivering nerve agents likely for assassination.

And part of this programme has involved producing and stockpiling quantities of novichok. This is a violation of the Chemical Weapons Convention.

The start of the investigation by the international chemical weapons watchdog comes as Johnson travels to Brussels to brief foreign ministers from across the European Union on the attempted assassinations in Salisbury before meeting with the NATO Secretary General.

An FCO spokesman said: "More than 20 countries across six continents have expressed their solidarity with us and we will continue to work with our European partners and allies around the world to tackle the threat posed by Russia to our collective security."

Background on 'Novichok'

Novichok is a nerve agent, which meets the Chemical Weapon Convention’s (CWC) definition of a chemical weapon, toxic chemical and precursor. Thus it is covered by the Convention’s prohibitions.

The chemical was positively identified by world-leading experts at the Defence Science and Technology Laboratory at Porton Down, which is one of the accredited and designated labs in the OPCW laboratory network.

Russia is the official successor state to the USSR. As such, Russia legally took responsibility for ensuring the CWC applies to all former Soviet Chemical Weapons stocks and facilities.

Members of the Convention must declare chemical weapons stockpiles and facilities and destroy Chemical Weapons within ten years of the convention coming into force for that party. In 1997, the Chemical Weapons Convention entered into force for Russia and solidified Russian commitments to chemical weapons disarmament and non-proliferation.

Developing, stockpiling and using chemical weapons are all a breach of the Chemical Weapons Convention.

While a diplomatic row over the incident in Salisbury rages on, a host of elaborate theories have emerged of exactly how the pair may have been attacked.

We have already heard a wide range of ideas concerning how the Novichok was administered - from being dispensed via an aerosol spray to being hidden inside Yulia's suitcase.

And now, it has been claimed a miniature drone - specially designed for assassinations - could have been used to poison the 33-year-old and her 66-year-old former double agent dad.

To read more about the latest theories and conspiracy theories, click here .