Mr. Skripal remains hospitalized almost five weeks after the poisoning, but his daughter, Yulia, has recovered and was moved this week to a secure location.
Claiming that the substance used in the incident had been a nerve agent allegedly developed in Russia, London rushed to accuse Moscow of being involved in the case without presenting any evidence.
Referring to the 2006 death of former KGB agent Alexander Litvinenko, Sedwill said Russian Federation had the operational means to carry out such an attack against Skripal.
Besides, the poisoning of Alexander Litvinenko in 2006 was mentioned in the letter as well.
Russia monitored emails for at least five years of the daughter of a Russian former spy who was poisoned in England last month, a British intelligence official said in a newly-released document.
Palestinians wounded at Gaza-Israel border protests
On Friday, most of the demonstrators assembled at five tent camps located several hundred meters (yards) from the border fence. UN Secretary-General Antonio Guterres and the European Union have called for an independent investigation into the violence.
"The code word used for the offensive chemical weapons programme (of which the Novichoks were one part) was FOLIANT", he said.
Sedwill goes on to claim that Russian Federation was likely the only former Soviet republic to pursue "an offensive chemical weapons programme after independence". "Within the last decade, Russian Federation has produced and stockpiled small quantities of Novichoks under the same programme".
"I would like to share with you and Allies further information regarding our assessment that it is highly likely that the Russian state was responsible for the Salisbury attack", Sedwill wrote in the letter.
"There is no plausible alternative explanation".
It published the unusual allegations in an 8,000-word report on the incident after the Kremlin was accused of using a military grade nerve agent in Salisbury, on March 4.
No, The Kardashians Haven't Unfollowed Tristan Thompson
The People sourced also called Thompson a "serial cheater" and expected more women to come forward in the coming days. Thompson was spotted at a club in New York City on Saturday, April 7 with a brunette - Instagram model Lani Blair .
Sergei is recovering more slowly than his daughter but will be able to leave Salisbury District Hospital in due course, said medical director Dr Christine Blanchard, earlier this week.
Alexander Yakovenko also accused the British authorities of destroying evidence in the Salisbury attack and said the British government had yet to produce the evidence to back its claims that Russian Federation was responsible.
Meanwhile, Russia's ambassador has accused the British authorities of destroying evidence in the Salisbury nerve agent attack.
He questioned the authenticity of a statement issued by Ms Skripal through the Metropolitan Police saying she did not want to meet Russian officials.
British Prime Minister Theresa May has said it is highly likely that Moscow was behind the attack although Moscow denies any involvement.
Harvey, Maria and others retired as hurricane names
You won't be seeing Harvey, Irma, Maria or Nate as names for hurricanes or tropical storms anytime soon. The storm stalled there for four days, causing catastrophic flooding in parts of southeastern Texas.
"We can not be sure that Yulia's refusal to see us is genuine".
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