Gary Aitkenhead, chief executive at Porton Down, told Sky News it was "not our job" to specify the source of the poison.
Russian Federation has already retaliated in kind after Britain initially expelled 23 diplomats.
He said that the creation of the Novichok agent would have required "extremely sophisticated" methods likely only to be available to a state actor.
He added: "we are continuing to work to help to provide additional information that might help us get closer to [the source] but we haven't yet been able to do that".
The global chemical weapons watchdog agency has said it will meet Wednesday to discuss Britain's USA -backed allegation that Russian Federation is responsible for the poisoning of a former double agent in England.
The head of the Porton Down military research facility has said his scientists have not verified that the nerve agent used in Salisbury came from Russian Federation.
The US ambassador was also ordered to shut the consulate in St Petersburg, in Russia's retaliation for the biggest expulsion of diplomats since the Cold War.
Officials believe this type of nerve agent attack is also meant to send a strong warning to Russian dissidents in the West, the source said.
A spokesperson said: "We have been clear from the very beginning that our world leading experts at Porton Down identified the substance used in Salisbury as a Novichok, a military grade nerve agent".
Watch Jesus Christ Superstar Live online
If you followed along with Chrissy , she kept commenting on how numerous characters were wearing deep V-neck shirts. Chrissy responded to John's tweet, "The deeper the V the closer to god". "She makes singing seem so easy".
Russian 'Satan 2' Missile Test Footage Released by Putin
Vladimir Putin kicked off March by nuking Florida in a demonstration video to boast about Russia's new "invincible" weapons. The Russian military says it has successfully tested its latest intercontinental ballistic missile.
Meanwhile, the ex-spy remains in a coma and in a critical condition but last week doctors said that his daughter had shown signs of recovery.
Russia's envoy to the European Union says British accusations of Russian involvement in the poisoning of an ex-Russian spy could have been an attempt to distract public attention from Britain's troubled exit from the EU.
The comments come a day ahead of an extraordinary meeting in The Hague of the executive council of the Organisation for the Prohibition of Chemical Weapons to discuss the Salisbury attack.
Mr Lavrov said it was "outrageous" that Britain had failed to provide consular access to Yulia Skripal, 33, since it emerged that her condition was improving.
Evgeny Buzhinsky, a retired lieutenant general, was quoted in British newspapers on Tuesday as saying the fallout of the attack could trigger "the last war in the history of mankind".
Russia's embassy in the United Kingdom said it "understood from the very start" that Whitehall statements on the nerve agent having been produced in Russian Federation were "a bluff".
Included among the science-related cargo is an experiment created to study severe thunderstorms on Earth. NASA mimicked atmospheric conditions to grow these tomatoes under LEDs rather than natural sunlight.
The final phase of the offseason program begins on May 22, when the Vikings will hold the first of 10 organized team activities. Phase Two - the following three weeks - includes on-field workouts limited to individual player instruction and drills.
The decision to walk Donaldson was reasonable on paper: He had hit two home runs in nine plate appearances against Robertson. NY is also 4-1 in Montgomery's last five starts and the under is 4-0 in their last four in game one of a series.
Villanova went on to beat North Carolina for the national championship with a three-point buzzer beater by Kris Jenkins. They play so well together. "When a team plays that well like they did tonight - Villanova - they just lit it up".
Born in 1936 in what is now known as the Eastern Cape province, Nomzamo Winifred Madikizela was the daughter of a history teacher. But her key role in sustaining the early anti-apartheid struggle was stained later by a reputation for coercion and violence.
Madhya Pradesh chief minister Shivraj Singh Chouhan tweeted his condolences and said: "We are monitoring the rescue effort". The identities of the other six deceased, including the two women, were yet to be established.
The U.S. exported $140.5 billion of agricultural goods last fiscal year, according to data from the U.S. The Dow Jones Industrial Average fell as much as 758 points on Monday, partly due to the trade spat.
Leonard spent three weeks in NY in February consulting with doctors and working out at the players union's facilities. The Kawhi Leonard-San Antonio Spurs saga has continued to unfold as the regular season slowly comes down to an end.
The S&P 500 index slipped 7.62 points, or 0.3% to 2,605, with six of its 11 main sectors ending in negative territory. GameStop ( GME.N ) shares fell 10.8 percent after the company provided disappointing full-year sales forecast.
Kashmir has been divided between India and Pakistan since the end of British colonial rule in 1947, but both claim it in full. Twenty people were killed in India-held Kashmir in some of the fiercest clashes this year in the region on Sunday.
While damaging winds will be our main severe weather threat, there is also a very small threat of some hail and a brief tornado. At this time, we will just have to wait and see if the cluster of storms survives and reaches our area late this afternoon.
Planning for next year's Easter at Coolidge has already started. "I had plans, but in the end I forgot about it", John Britt said. Kids raced around the yard near 950 Field Drive in Noblesville looking for eggs filled with candy and tickets to win prizes.
Disney Could Acquire Sky News in Fox-Sky Deal's Latest Twist
In this scenario, Disney would end up acquiring Sky News whether or not its purchase of Fox's entertainment assets goes ahead. Mr Murdoch's family already have extensive media holdings, including stakes in newspapers such as The Sun and The Times .
Oklahoma Teachers March on Capitol
As discontent spread, the teachers organized a statewide, 55-county strike that lasted nine days, ending when Republican Gov. Educators in Oklahoma are fed up with years of budget cuts and salaries that rank among the lowest in the country.
Turkey ready for new targets outside borders: Erdogan
Investments will be made in projects that strengthen trade, economic and investment ties between Russian Federation and Turkey ". The number of power plants under construction will be 56, including Akkuyu plant.
Fortnite Vending Machines: What to Expect
The iOS version is the same as the console and PC versions, and it includes cross-play across all platforms. The good news is there is cross play, so iPhone gamers can play with and against PS4 and Xbox players, too.
Google will bring low-priced Pixel, Home speakers to India
Rivals like Xiaomi, Samsung, and Motorola have already partnered with several multi-branded retail chains across India. The smaller Google Home mini is much more affordable with a price of $49 which is almost Rs 3,100 plus on conversion.
Corbyn on Labour anti-Semitism row
It carried the caption: "When you're pictured at Nuremberg and claim you thought you were going to a vehicle rally". After it was called out as distasteful by many outraged Twitter users, the Apprentice host has since deleted it.
Trump slams immigrant 'caravans', says 'DACA is dead'
This morning, Trump also casually demanded an end to the filibuster, so the Senate could pass border legislation he wanted. The official claimed any politician who opposes the changes the White House seeks is supporting illegal immigration.
Thousands pay their respects at Stephen Hawking's funeral
Mike Meylan, a New Zealand mathematics professor visiting Cambridge University, brought his children to explain Hawking's genius. Today, his coffin was driven via a hearse through Cambridge , topped with white "Universe" lilies and white "Polar Star" roses.
Google Shuts Down Goo.gl URL Shortening Service
For example, deep links offer the benefit of being able to redirect someone to not only a website, but a point within a website. While this could be upsetting, it really isn't that big of a deal to the numerous other URL shorteners that are available.
Fister outduels Keuchel as Rangers beat Astros
BREWERS 2, PADRES 1: Orlando Arcia singled in Ji-Man Choi with two outs in the 12th inning to lift Milwaukee at San Diego. Fister is 1-1 with a 3.44 ERA (seven earned runs in 18.1 innings pitched) over three career starts versus Houston .