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By Aubrey Allegretti, News Reporter

Sergei Skripal's suspected poisoning has thrown up concerns again about Russian spies dying on British soil in mysterious circumstances.

The former military intelligence colonel is fighting for his life after falling unconscious in a shopping centre in Salisbury.

But, as UK security services and forensic officers investigate the incident, Labour's shadow home secretary Diane Abbott warned: "We cannot allow London and the Home Counties to become a kind of killing field for the Russian state."

Her words follow the Russian-linked deaths of several spies in Britain, and threaten to put further strain on diplomatic tensions between London and Moscow.

Suspected killings include:

Georgi Markov

The Cold War burst onto the streets of London in 1978 when Bulgarian dissident Georgi Markov was shot with a poison dart on Waterloo Bridge.

He recalled feeling a prick on the back of his leg, and seeing a man hurriedly pick up an umbrella and dive into a taxi.

Mr Markov continued into work and complained to a colleague about a small red pimple that had formed at the site of the sting.

He died four days later at a hospital in south London.

An autopsy found a metal pellet lodged in Mr Markov's leg but no-one has since been charged with his murder.

A former KGB official in charge of the then-Soviet Union's spy agency, Oleg Kalugin, said years later he sent two operatives to Sofia to provide Bulgaria with the pellet used.

Gareth Williams

Welsh mathematician and MI6 employee Gareth Williams was found dead and naked in a locked bag inside his south London flat in 2010.

An inquest found the death was "unnatural and likely to have been criminally mediated", while a subsequent Met Police inquiry claimed it was "probably an accident".

But a former KGB major, Boris Karpichkov, claimed Williams was murdered by Russian hitmen who blackmailed him with compromising sexual photographs.

No prints or DNA belonging to the 31-year-old were found on the rim of the bath he was discovered in.

Mr Williams' family maintain he was murdered.

Alexander Litvinenko

The case that could share the closest parallels with Mr Skripal's is that of Alexander Litvinenko - who was poisoned with a cup of tea.

A former FSB officer granted political asylum in Britain, Mr Litvinenko fell ill after sipping from a cup laced with radioactive polonium at a meeting in central London in 2006.

He was admitted to University College Hospital and survived for 23 days, managing to give a warning to Russia.

He said: "You may succeed in silencing one man but the howl of protest from around the world, Mr Putin, will reverberate in your ears for the rest of your life."

Russia has denied any involvement and refused to extradite two men authorities believe are behind the murder.

Boris BerezovAG百家乐在线官网

Another Russian oligarch and dissident who fled to Britain and died was Boris BerezovAG百家乐在线官网.

He was discovered at his home in Berkshire in a locked bathroom with a ligature around his neck.

At the inquest, his daughter raised a report by a German pathologist which contradicted that carried out by the Home Office, calling it strange that the ligature mark on her father's throat was circular, rather than v-shaped, and that his face was purple, rather than white.

The coroner said it was "impossible to say" whether Mr BerezovAG百家乐在线官网 took his own life or was killed.

Later, Moscow published a letter from him to President Putin asking for "forgiveness for his mistakes", which his girlfriend corroborated.

Scot Young

A Scottish property developer, who is believed to have had links to Mr BerezovAG百家乐在线官网, also died in suspicious circumstances.

Scot Young fell to his death from a window in his ex-girlfriend's London flat, becoming impaled on the railing below.

He claimed to have lost all his money in divorce proceedings, but Scotland Yard later obtained a dossier of phone calls between the pair that showed he had links to Mr BerezovAG百家乐在线官网.

After he died in 2014, it was discovered he had been diagnosed with bipolar disorder and been treated for "drug and alcohol abuse".

A subsequent investigation by BuzzFeed claimed US spy agencies believed Russia was involved in the death.

Alexander Perepilichnyy

Russian businessman Alexander Perepilichnyy collapsed and died while out jogging near his expensive home in Surrey.

Before his death, he was investigating a Russian money-laundering operation worth $230m, an inquest was told.

Originally, the death was attributed to natural causes - but traces of a chemical sometimes found in the poisonous plant gelsemium were discovered in his stomach.

The firm he worked for, Hermitage Capital Management, claimed he would have been killed for helping it uncover the scam involving top-level Russian officials.