Sergei Skripal escaped Russia — but knew he could never outrun the Kremlin.

The former Russian army officer-turned-British double agent’s life changed instantly in 2010 when he was plucked from a brutal penal colony where he was serving time for treason and flown to the UK in the high-profile prisoner swap that saw sexy spy Anna Chapman, who had been living in New York, deported to Russia.

Skripal left a punishing existence behind bars under Vladimir Putin’s thumb for a comfy retirement, and lived the next seven years in a sleepy English town on a government pension.

But he could never stop looking over his shoulder, according to family members.

“From the first days, he knew that it could end badly and that he would not be left alone,” an anonymous relative told BBC Russia.

On Sunday, Skripal’s past may have finally caught up with him, when he and his daughter, 33-year-old Yulia, suddenly collapsed on a bench outside a local shopping mall — after exposure to what authorities say was an “unknown substance.”

Adding to the intrigue, Skripal’s wife, son and brother have all passed away over the past five years — and family members remain suspicious about the circumstances, according to the BBC.

His wife, Lyudmila, reportedly died of cancer in the UK in 2012, his older brother passed away in Russia two years later, and his son, Alexandr, died of liver failure last year while on holiday in the Russian city of St. Petersburg.

On Tuesday, Skripal and Yulia were fighting for their lives in a hospital while British authorities scrambled to figure out how they fell ill — and whether Moscow was finally exacting revenge for the military officer’s betrayal.

The parallels to the 2006 assassination of defected Russian agent Alexander Litvinenko — who was poisoned with radioactive tea in London, likely on the orders of Putin himself, an inquiry recently found — have not gone unnoticed by Britain’s leaders.

The case is so “unusual,” Scotland Yard said Tuesday, that it is placing its counterterrorism police in charge of the investigation — while UK Foreign Secretary Boris Johnson threatened “robust” retaliation if the Kremlin is found to have any connection to Skripal’s illness.

“Should evidence emerge that implies state responsibility, then Her Majesty’s government will respond appropriately and robustly,” he told Parliament, adding that such a response could include boycotting the upcoming soccer World Cup in Russia.

“I think we will have to have a serious conversation about our engagement with Russia, and for my own part, I think it will be very difficult to see how we can,” Johnson said.

A Kremlin spokesman called it a “tragic situation” but denied any knowledge of the incident in Moscow.

But Putin himself declared amid the 2010 prisoner swap that “traitors always end badly.”

“Secret services live by their own laws and these laws are very well known to anyone who works for a secret service,” the former KGB officer said ominously at the time.

An elaborate poisoning plot would certainly be a fitting climax for a man whose life story reads like a Cold War thriller.

A decorated former military man, Skripal was working for Russia’s Foreign Ministry when he was arrested in 2004 — after authorities say he was caught leaking information about Russian military spies to the UK via a James Bond-style transmitter hidden inside a rock in a Moscow park, the Daily Mail reported.

They say he was paid more than $100,000 for the intel.

Skripal was sentenced to 13 years behind bars, and served time in one of Mordovia’s notoriously tough prisons until 2010 — when he and three others were exchanged for 10 members of a Russian sleeper cell that had been uncovered in the United States.

Among the spy ring’s ranks was Chapman, the flame-haired daughter of a Russian diplomat who was working as a real estate agent in New York — and has since parlayed her notoriety and bombshell looks into a post-espionage career as a media personality.

Skripal, however, settled into a quiet, low-key life in the small city of Salisbury with Lyudmila and Yulia. He bought a red-brick house and a BMW, liked to play the lottery and recently joined a social club, the Guardian reports.

“He said hello if he walked past, and seemed like a nice chap,” neighbor James Puttock told the paper. “When he moved in, he invited us all over for a housewarming party; I imagine he invited the whole street.”

When Lyudmila died in 2012, Yulia reported her passing to the local authorities and described her father as a former government planning officer, according to the paper.

Yulia, who had worked for Nike in Russia before the 2010 move, was employed at a Holiday Inn in England as recently as 2014, but more recently seems to have moved back to Moscow, where she works for PepsiCo Russia, according to her Facebook page.

But she regularly visits her dad in Salisbury, photos on the page suggest, and their housekeeper told the Guardian she was on one such trip when the pair were suddenly afflicted with a mystery illness in the middle of town on Sunday afternoon.

“She was slumped over on the man’s shoulder,” Freya Church, 27, who found the pair on a bench, told the Telegraph.

“To be honest, I thought they might be homeless but they were perhaps better dressed,” Church said. “She had a red bag at her feet. He was gesturing at the sky, doing some kind of movements with his hands . . . He was looking up and his eyes were glazed.”

Investigators in anti-contamination suits later sealed off the site and several nearby businesses — including two restaurants.

On Tuesday night, cops were still outside the eateries — Italian chain Zizzi and pub The Mill — and were preparing to go inside, according to local paper the Salisbury Journal.

Meanwhile, radiation and toxicology experts have been tapped to help work out exactly what the Skripals have been exposed to.

Nothing untoward appears to have happened to the other three Russians swapped alongside Skripal — Igor Sutyagin, Aleksandr Zaporozhsky and Gennady Vasilenko.

“I am not concerned,” Sutyagin told the Times of London, adding that he’s not sure Russia is to blame. “There are lots of former security officers that deserted to the West.”

But Marina Litvinenko, the widow of assassinated spy Alexander Litvinenko, says the whole situation feels suspiciously familiar to her.

“It’s like deja vu, [like] what happened to me 11 years ago,” she told BBC Radio.

“In Russia it is still [an] old-fashioned and old-style KGB system,” she added. “It’s still all the same. If there is an order to kill somebody, it will happen.”