Police looking into the poisoning of a former Russian spy and his daughter say it's a "complex investigation".

The British Home Secretary says the Salisbury poisoning investigation is progressing "with speed".

More than 250 counter-terrorism officers are looking into who targeted former Russian spy Sergei Skripal and his daughter Yulia a week ago.

The Kremlin has denied any involvement.

Amber Rudd was asked if the UK government was any clearer on who was behind the attack.

"It's too early for that because it's a really painstaking and detailed investigation as the police go through over 200 pieces of evidence, huge amounts of CCTV.

But they are absolutely committed to do that in a completely professional way."

Up to 500 people who visited two restaurants in Salisbury in the UK are being urged to wash their clothes after traces of a suspected nerve agent were found.

Former Russian spy Sergei Skripal and his daughter remain critically ill.

Dame Sally Davies, England's chief medical officer, claims the risk to the public is low.

But says some people are concerned prolonged exposure may, over weeks, cause health problems.

"I'm therefore advising, as a bolt and braces approach, people who were either in Zizzi's restaurant or the Mill Pub from 1.30pm last Sunday until evening closing on Monday should clean the clothes they wore and the possession they handled while there."

The widow of ex-Russian agent Alexander Litvinenko, claims lessons haven't been learned following his murder - after traces of a nerve agent were found in Salisbury.

Marina Litvinenko - whose husband was poisoned in London more than a decade ago - says there needs to be tougher sanctions to prevent a similar attack.