Russia denied the attack. Russian President Vladimir Putin labelled Sergei Skripal a "traitor" and a "scumbag" and insisted the suspects were civilians.
Senior UK government sources have told The Sunday Times the Skripals have been given new identities to start a life in New Zealand after a year in an MI6 safe house.
Former Russian military intelligence colonel Sergei Skripal at the Moscow District Military Court in Moscow in 2006.
Former UK ambassador to Russia Sir Tony Brenton said it made sense for Britain to send Skripal to New Zealand where it may be harder for Russia to pursue him, if they still wanted to.
"I was surprised because I assumed they were going to hide him away somewhere here in the UK which is where after all he's used to living.
"But on the other hand it makes sense - it was going to cost an awful lot of money here to keep him permanently protected against potentially other Russian attempts to get at him.
"I'm afraid that from New Zealand's point of view it means that while I assume he's quite carefully concealed, you have a potential security problem on your hands if the Russians do indeed want to track him down and get at him."
Skripal, a former Russian intelligence officer, was arrested in 2004 and put on trial for "high treason in the form of espionage" by a Moscow military court.
Yulia Skripal speaks to media in London, in May 2018.
Authorities accused him of being a double-agent who shared Russian state secrets with Britain's MI6.
He confessed and co-operated at trial and was then jailed for 13 years, but was pardoned in 2010 then released as part of a prisoner exchange.
Sir Tony said it was a mystery as to why the Russians went after the Skripals as they do not usually attack people returned in prisoner swaps. He said the "sheer glaring incompetence" of the operation led some to conclude it was a warning operation to other dissidents in the UK and if that were true Russia was less likely to go for him again.
"The worry is they're still after him for reasons we don't know, in which case if they find him and if they get to him you have a huge row with Russia on your hands."
Security analyst Paul Buchanan said it was possible the story was "planted" to divert attention and the Skripals are actually hiding somewhere else.
Reports emerged in February that MI6 was looking to the US, Australia and New Zealand, he said.
"That tells me it could be a diversion. It's possible they'll stay in the UK or move to another Commonwealth country.
"Reports in February were very specific, that they would move to a Five Eyes country because they could not be protected in the UK. Although I remain sceptical because of that, and because of the way this news has built up, it is plausible that they would come here.
If they were in New Zealand it would be difficult for them to stay anonymous, he said.
"They'll be very very visible in the main centres where Russians congregate in New Zealand."
Both had strong Russian accents layered into a British accent, he said, and would have to be located somewhere there was no Russian community.
Sir Tony Brenton said it was possible for Skripal to be safe in New Zealand - though it was worrying the story had become public.
"On the assumption he's in New Zealand your security authorities will know where he is and will keep a general eye, but will be very much hoping that's a routine thing that doesn't turn into an actual need to secure him.
"Provided that he sticks to his cover properly and provided that the Russians are not seriously pursuing him any more I think he's almost certainly reasonably safe. The worrying factor is that it has become public that he is in New Zealand."
Sir Tony said it was quite unusual to have a change of country - and this would be the first sharing arrangement he had heard of.
