Britain will methodically work out who carried out a nerve agent attack on a Russian ex-spy and his daughter in a quiet English city, then take action, interior minister Amber Rudd said on Thursday.

Former double agent Sergei Skripal, 66, and his daughter Yulia, 33, have been in hospital in a critical condition since they were found unconscious on Sunday on a bench outside a shopping center in Salisbury, according to Reuters.

British media and some politicians have speculated that the Russian state could be behind the attack - suggestions dismissed by Moscow as part of an anti-Russian campaign.

 “When we have all the evidence of what took place we will, if it is appropriate, attribute it to somebody,” she said during an interview on BBC radio.

“If that is the case then we will have a plan in place. We need to be very methodical, take a cool head and be based on the facts, not on rumor,” she said.

Read alsoRussian FSB earlier used substance that poisoned ex-Russian spy SkripalPolice said on Wednesday that a nerve agent was used against Skripal and Yulia. A British police officer who was also harmed by the substance was now able to talk to people although he remained in a serious condition, Rudd said.

Scientific tests by government experts have identified the specific nerve agent used, which will help identify the source, but authorities have refused to disclose the details.

Rudd was pressed during her BBC interview on whether Britain had been too soft on Russia following the murder in London in 2006 of ex-KGB agent Alexander Litvinenko, sending out a message that such acts could be carried out with impunity.

She strongly denied this and hinted that if Russia turned out to be implicated in the attack on Skripal, action would be taken against it.

“We are absolutely robust about any crimes committed on these streets in the UK. There is nothing soft about the UK’s response to any sort of state activity in this country,” she said.

“We know exactly what we’re doing. We are going to be waiting for the evidence. When we have it, if we attribute, we will have a plan.”