Russian Foreign Minister Sergei Lavrov suggested on Monday that the British government might have poisoned Sergei Skripal to cover up difficulties over Brexit.

Britain has insisted the Russian government was behind the nerve-agent poisoning of the former Russian spy and his daughter March 4 in the English city of Salisbury, a charge the Russians vehemently deny.

Sergei Skripal, left, and his daughter Yulia were found unconscious on a park bench in Salisbury, in southwest England, last month. Yulia is no longer in critical condition. (Misha Japaridze/AP; Yulia Skripal/Facebook via AP)

Russian officials and state television have in turn come up with several different theories to explain the poisoning.

At a news conference in Moscow on Monday, Lavrov denounced the British accusation as a "mad and horrible provocation." He repeated a suggestion previously made by Russia that British secret services were the ones who deployed the nerve agent Novichok.

Sergei Lavrov suggests poisoning may have been to distract public from Brexit issues 1:33

"There are other explanations besides those put forward by our Western colleagues who declare that it can only be the Russians who are responsible," he said. "There are other explanations; experts say that it could be highly advantageous to the British security services as well, who are well known for their capacity to act with a licence to kill.

"It could also be advantageous to the British government, who clearly find themselves in a difficult situation, having failed to fulfil their promises to voters over Brexit."

Twitter campaign

The statements come as the Russian Embassy in London put out a series of tweets over the weekend raising issues with the British response and investigation into the Skripal poisonings. The 14 questions tweeted out cover everything from how U.K. experts determined the nerve agent originated in Russia, to how medical personnel would have had antidotes available.

"Why has Russia been denied consular access to the two Russian nationals, Sergei and Yulia Skripal, that have become crime victims in the British territory?" the first of the tweet read.

A number of the tweets raise allegations that France was closely involved in the British investigation.

"On what grounds has France been involved in technical co-operation with regard to the investigation of an incident in which Russian nationals had suffered?" one of the questions says.

"Were French experts present when biological material was taken from Mr. and Ms. Skripal?" another one asks.

Question without answer #13: Does the UK possess reference samples of the military-grade poisonous substance that British representatives identify as “Novichok”? <a href="https://t.co/Su7gku4OxS">pic.twitter.com/Su7gku4OxS</a> —@RussianEmbassy

Question without answer #7: What evidence has been passed to France for studying and/or for a French investigation? <a href="https://t.co/0FM4w2KeX8">pic.twitter.com/0FM4w2KeX8</a> —@RussianEmbassy

British police said last week that the Skripals might have been exposed to Novichok at the front door of their home. A statement by Scotland Yard said specialists have identified the highest concentration so far of the nerve agent as being "on the front door of the address."

Sergei, 66, and Yulia Skripal, 33, were both in critical condition after they were found unconscious on a public bench in Salisbury and taken to hospital, but Yulia Skripal's condition has improved and her condition is no longer considered critical.

Sergei Skripal was a Russian military intelligence officer when he began passing state secrets to Britain's MI6 agency in the mid-1990s. He was eventually caught and tried in Russia in 2006, and was found guilty and imprisoned. But in 2010, Russia released him to the U.K. as part of a prisoner swap for former Russian spies.

Lavrov argued Monday that Russia had no motive to attack Skripal. "If there were any gripes against the man, he wouldn't have been swapped," Lavrov said.

Skripal had been living a low-profile life in Salisbury, in southwest England.

Lavrov said that Russia has called a meeting of the international chemical weapons watchdog for Wednesday to discuss the case and asked it to provide details of its co-operation with Britain in the poisoning probe.

Alexander Shulgin, the Russian representative at the Organization for the Prohibition of Chemical Weapons (OPCW), said Monday in televised remarks that the organization must conduct an "open, thorough and unbiased investigation" with the participation of Russian experts.

British authorities invited OPCW experts to take chemical samples from Salisbury, the English city where the father and daughter were sickened, and to analyze them. Shulgin warned that Moscow won't accept the agency's conclusions unless Russian experts are allowed to take part in the process.