Russia will have committed a "brazen act of war" against Britain if Moscow is found responsible for a nerve agent attack on UK soil, the Government has been told.
The poisoning of Russian double agent Sergei Skripal, 65, and his daughter Yulia, 33, in Salisbury, Wiltshire, was described by Home Secretary Amber Rudd on Thursday as "attempted murder in the most cruel and public way".
The Cabinet minister promised a "robust" response once responsibility for the attack is established, but she told the House of Commons her current priority is on the immediate response to the incident.
However, senior Tory MP Sir Edward Leigh used Ms Rudd's update to MPs on the poisoning to demand increased defence spending.
He declared the Government's two per cent of GDP spend on defence as "not enough" in the face of the current threat from Russia.
Sir Edward also claimed there is "very strong" circumstantial evidence against Moscow over the attack on former spy Mr Skripal and his daughter.
"Who else would have the motive and the means?" he asked.
"Those of us who seek to understand Russia know that the only way to preserve peace is through strength.
"And, if Russia is behind this, this is a brazen act of war. Of humiliating our country."
Ms Rudd told Sir Edward "there will come a time for attribution", which would subsequently lead to "consequences".
Earlier, in a statement to MPs, Ms Rudd called the use of a nerve agent on UK soil as "a brazen and reckless act".
She added: "People are right to want to know who to hold to account. But, if we are to be rigorous in this investigation, we must avoid speculation and allow police to carry on their investigation.
"The investigation now involves hundreds of officers following every possible lead to find those responsible.
"We are committed to doing all we can to bring the perpetrators to justice - whoever they are and wherever they may be.
"The investigation is moving at pace and this Government will act without hesitation as the facts become clearer."
Earlier on Thursday, Ms Rudd revealed to Sky News a police officer who was hurt in the attack is "awake and engaging with people", although Mr Skripal and his daughter remain in a "very serious" condition in hospital.
Her fellow Cabinet minister, Defence Secretary Gavin Williamson, also declined to say whether Moscow was responsible for the Salisbury attack but branded Russia an "ever-greater threat" to the UK.
"Russia's being assertive, Russia's being more aggressive, and we have to change the way that we deal with it," he told ITV.
"We can't be in a situation in these areas of conflict where we are being pushed around by another nation."
Former Tory minister Nick Boles called on the Government to "turn tough talk into action" over Russia, posting on Twitter: "I do not see how we can maintain diplomatic relations with a country that tries to murder people on British soil and puts the lives of British citizens at risk."
Former top UK diplomat Sir Christopher Meyer, who previously worked in Moscow, said any evidence of Russian state involvement in the Salisbury poisoning would require a British response.
He told Sky News: "You cannot tolerate a government assassination on British soil. It is absolutely beyond the pale and needs a reaction."