In her statement, she said that she has been made aware of the Russian embassy's attempts to contact her but she did not wish to avail of their services for the moment.
"We have information indicating Russian intelligence service interest in the Skripals dating back at least as far as 2013", he said.
Russian Federation later ridiculed the number of theories over the poisoning in western media.
The UK's defence laboratory at Porton Down had established the highest concentrations of the poison were found on the handle of Sergei Skripal's front door, the letter said. They must be able to perform off-site analysis of chemical samples collected by OPCW inspectors from chemical production facilities, storage depots and other installations, or from the site of an alleged use of chemical weapons.
The U.K. has said its experts from the Porton Down defense research center concluded that the military-grade nerve agent that was used in the attack belonged to a Russia-developed chemical weapon group called Novichok.
The chemical weapons watchdog did not explicitly name Novichok in its published summary, say where the poison may have come from or assign blame for the attack.
Theresa May stressed that the best hope for the Syrian people is a political solution, with the air strikes launched by W western countries on Syria being essentially "about deterring the barbaric use of chemical weapons in Syria and beyond".
Moscow has strongly denied responsibility and says Britain is waging a defamation campaign against it.
Russian Federation has said it does not have such nerve agents and President Vladimir Putin said it was nonsense to think that Moscow would have poisoned Skripal and his daughter.
"The program subsequently include investigation of ways of delivering agents, including by application to door handles".
The Kremlin has also been linked to the deaths of numerous prominent Russian dissidents and Mr Sedwill said "Russia has a proven record of conducting state-sponsored assassination".
Almost a month after the pair were found unconscious on a bench in Salisbury, Yulia has been discharged from hospital and her father Sergei is no longer in a critical condition.
The UK believes it is highly likely that the Russian State orchestrated the attack. Sergei Skripal was imprisoned in Russian Federation for selling secrets to British intelligence in the 1990s but was exchanged in a spy swap in 2010.
"There is no plausible alternative explanation".
Britain said the report's characterization of the substance as highly purified supported its stance that the chemical could have been produced only by a state laboratory, although Moscow refuses to provide samples against which it could be matched.