The National Crime Agency is leading an investigation into a suspected cyber attack on a British institute that seeks to counter Russian disinformation, Sky News can reveal.

Whitehall sources said it is highly likely that Russia's military intelligence service carried out the hack-and-leak of files from the little-known Institute for Statecraft.

The move may have been in response to Britain implicating the GRU in the Salisbury spy poisoning last year and pledging to "shine a light" on the agency's covert activities, they said.

Security officials regard the attack as significant because - if confirmed - it would mark the first time Russia has conducted a hack-and-leak operation in Britain, a separate source said.

The GRU is suspected of deploying such a tactic against the Democratic National Committee in the 2016 US presidential elections and against the World Anti-Doping Agency.

Chris Donnelly, a former senior Ministry of Defence civil servant who co-founded the Institute for Statecraft, said he has little doubt who hacked it, though he has no "forensic proof".

"Our assessment… is that this is the GRU," Mr Donnelly, 72, told Sky News in an interview.

He said the launch of the investigation by the National Crime Agency, elite law enforcement with the power to investigate cases with an overseas dimension, is important.

It "demonstrates that the government clearly considers this to be a significant attack by a hostile foreign power against the UK," Mr Donnelly said.

But Dmitry Peskov, President Vladimir Putin's spokesman, denied any Russian involvement.

"We have been saying numerous times that Russian state has nothing to do with any cyber attacks, to the contrary we have been offering international cooperation to combat cyber-crime but unfortunately we met no reciprocity from our counterparts," Mr Peskov said in an emailed response to a question about the allegations.

Files stolen from the institute - registered in Scotland as an educational charity but based in London and with about 12 staff - have been leaked online every few weeks since November.

Kremlin-funded media outlets have used the leaks to accuse Britain of conducting "shadowy" information operations to stoke anti-Russian sentiment around the world.

A number of individuals - including institute staff - have had their names, bank details, home addresses and other private information leaked as part of the attack.

One said he suspected intruders had on a number of occasions broken into his home and moved objects around, including miniature figurines on a shelf.

He called the suspected home intrusions a concern but said this is the kind of tactic the Russian security services use against certain foreign diplomats, military personnel, journalists and others living in Moscow.

"It's all part of the bigger game of exerting psychological pressure to unsettle, deflect, divert focus," the institute employee said, asking to remain anonymous.

Mr Donnelly, who speaks Russian and is an expert on the Kremlin and Russian military strategy, said the hackers used sophisticated techniques, not leaving behind easy-to-spot tracks, which he said was a further indication of a Kremlin-sponsored attack.

"The only thing that makes sense is this is a Russian state attack against what we are doing," he said, speaking from his home in Cumbria.

The National Cyber Security Centre, part of GCHQ, has been investigating the suspected hack since late last year. The NCA's involvement began in recent weeks.

An NCA spokeswoman said: "We confirm we are conducting a criminal investigation into a suspected cyber attack against the Institute for Statecrft (IFS) and the subsequent release of information.

"As our investigation is ongoing, we cannot comment further at this time".

An entity that calls itself Anonymous, but offers no proof of being affiliated with the worldwide group of activist hackers, uploaded the stolen files on a third party website.

The first of six dumps was in early November. The most recent one was in early February.

They focus on an institute programme called the Integrity Initiative, which Mr Donnelly said aims to combat and raise awareness about disinformation used by Russia and others, including China and the Islamic State group, in Europe and elsewhere.

The programme has received more than £2.2m in funding over the past two years from the Foreign and Commonwealth Office, which is keen to expose fake news and propaganda being used by hostile states as a weapon to harm democratic societies.

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