Three suspected spies for Russia in the UK have been arrested and charged in a major national security investigation, the BBC can reveal.

The defendants, all Bulgarian nationals, were held in February and have been remanded in custody since.

It is alleged they were working for the Russian security services.

They are charged with possessing identity documents with "improper intention", and are alleged to have had these knowing they were fake.

The documents include passports, identity cards and other documents for the UK, Bulgaria, France, Italy, Spain, Croatia, Slovenia, Greece, and the Czech Republic.

The three defendants were arrested under the Official Secrets Act by counter-terrorism detectives from the Metropolitan Police, which has national policing responsibility for espionage.

They are:

• Orlin Roussev, 45, of Great Yarmouth, Norfolk
• Bizer Dzhambazov, 41, of Harrow, north-west London
• Katrin Ivanova, 31, of the same Harrow address

The trio have lived in the UK for years, working in a variety of jobs, and living in a series of suburban properties.

Mr Roussev has history of business dealings in Russia.

He moved to the UK in 2009 and spent three years working in a technical role in financial services.

His online LinkedIn profile states he later owned a business involved in signals intelligence, which involves the interception of communications or electronic signals.

Mr Roussev, whose most recent address is a seaside guesthouse in Great Yarmouth, also states he once acted as an adviser to the Bulgarian ministry of energy.

In Harrow, former neighbours described Mr Dzhambazov and Ms Ivanova as a couple.

Mr Dzhambazov is described as a driver for hospitals, and Ms Ivanova describes herself on her online LinkedIn profile as a laboratory assistant for a private health business.

The pair, who moved to the UK around a decade ago, ran a community organisation providing services to Bulgarian people, including familiarising them with the "culture and norms of British society".

According to Bulgarian state documents online, they also worked for electoral commissions in London which facilitate voting in Bulgarian elections by citizens living abroad.

Speaking to the BBC, neighbours at two houses previously occupied by the couple said they brought round pies and cakes as gifts.

At their most recent Harrow home, neighbours said detectives spent a significant amount of time searching it, with a visible police presence for over a week.

The three defendants are due to go on trial at the Old Bailey in London in January. They have yet to enter pleas to the charges.

Counter-terrorism police have spoken publicly about the increasing amount of time spent on suspected state threats and espionage, especially relating to Russia.

Their concern follows notorious incidents from recent years involving Russian intelligence operations in the UK.

In 2018, Russian operatives attempted to murder former double-agent Sergei Skripal and his daughter Yulia in Salisbury, Wiltshire, using the deadly nerve agent Novichok. The pair, as well as responding detective Nick Bailey, were treated in hospital and could have died.

Later that year, local woman Dawn Sturgess - who was unconnected to the Skripals - died after being exposed to the nerve agent, which had been left in Wiltshire in a perfume bottle.

In 2006, former Russian-intelligence officer Alexander Litvinenko was killed in London after being poisoned by assassins working for the Russian state.