A woman who was exposed to the nerve agent Novichok in southern England last week has died in hospital.

Dawn Sturgess, 44, and partner Charlie Rowley, 45, were found unconscious last week in Amesbury, Wiltshire – a few kilometres from the scene of a nerve agent attack on a former Russian spy and his daughter in March.

UK police have opened a murder investigation after Ms Strugess passed away on Monday morning (AEST). Mr Rowley remains critically ill in hospital.

Police believed the pair had secondary exposure to the chemical weapon used in the attack on the Skripals.

In a statement, Assistant Commissioner Neil Basu, the head of UK counter-terrorism policing, said: “This is shocking and tragic news.

“Dawn leaves behind her family, including three children, and our thoughts and prayers are with them at this extremely difficult time.

“This terrible news has only served to strengthen our resolve to identify and bring to justice the person or persons responsible for what I can only describe as an outrageous, reckless and barbaric act.”

Mr Basu said police were still investigating what contaminated item the couple had touched.

Prime Minister Theresa May said she was “appalled and shocked” by Sturgess’ death.

“Police and security officials are working urgently to establish the facts of this incident, which is now being investigated as a murder,” she addded.

“The government is committed to providing full support to the local community as it deals with this tragedy.”

Ms Sturgess’ death comes after British Home Secretary Sajid Javid demanded that Russia provide details about the Novichok nerve agent used on the Skripal family.

Russia has denied any involvement in the March incident and suggested the British security services had carried out that attack to stoke anti-Moscow hysteria.

“It is now time that the Russian state comes forward and explains what has gone on,” Mr Javid told parliament last week.

“It is completely unacceptable for our people to be either deliberate or accidental targets, or for our streets, our parks and towns to be dumping grounds for poison.”

-with AAP