Britain was to hold a second meeting of its national emergencies committee on Saturday in response to a nerve agent attack on a Russian former spy.

Home Secretary Amber Rudd was to chair the meeting of the committee, known as COBRA, at 1500 GMT to receive updates on the police case, the government said.

The March 4 attack on Russian double agent Sergei Skripal and his daughter Yulia in Salisbury, southwest England, is being treated as attempted murder.

Around 180 troops, including chemical warfare experts, have been deployed in the city after investigators requested specialist assistance.

Prime Minister Theresa May has said Britain will react with the appropriate response if a state was found to be behind the murder attempt.

Authorities are racing to determine the source of the nerve agent used against Skripal, who came to Britain in 2010 as part of a spy swap.

Foreign Secretary Boris Johnson has been pointing the finger at Moscow.

Security Minister Ben Wallace told BBC radio on Saturday that the government stood ready to respond "with the full force of the UK's resources".

He added that the attack was "a very serious incident with horrendous outcomes".

"Someone has come onto our soil... has recklessly, brazenly, committed what looks like a very nasty crime, with a nerve agent prohibited, by most international laws... and has potentially put lots of people at risk," he said.

May was "expected to announce sanctions" against Russia "as soon as Monday", according to the Daily Telegraph newspaper.

The Times newspaper said British officials were talking to their US and European counterparts about a co-ordinated retaliation, which could include "diplomatic, economic and military measures" if Moscow was behind the attack.

- 'Time for cool heads' -

Skripal was a former colonel in Russia's military intelligence who was jailed in his country for betraying agents to Britain's MI6 secret service.

He was pardoned in 2010 before being flown to Britain.

In Salisbury, police widened their searches to places frequented by Skripal, 66, and his 33-year-old daughter.

Sergei Skripal bought groceries at a corner shop before collapsing -, AFP

The former double agent and his daughter were found slumped on a bench in the city.

They are in a critical but stable condition in Salisbury District Hospital.

Nick Bailey, one of the first officers on the scene, is also being treated but is sitting up and talking after initially being admitted to intensive care.

The road leading to Skripal's home was cordoned off Saturday, with police standing guard.

Meanwhile officials wearing biohazard protective suits were seen working alongside police officers at the London Road Cemetery, where Skripal's wife and son were laid to rest.

Officers were seen placing a blue forensic tent over his son's memorial stone before appearing to place items in several yellow barrels.

Government minister John Glen, the member of parliament for Salisbury, said a "whole range of tools are at our disposal" once it is established who was behind the incident.

"The government will not be acting precipitously in response to what has happened. Now is the time for cool heads and a rational examination of the facts," he said.

"Once these are established, then and only then, will an appropriate and proportionate course of action be taken. A whole range of tools are at our disposal depending on who has perpetrated this act, including a number of financial and economic levers."