A spokesman for Russia’s defence ministry accused the UK of being involved in staging the attack.
The US and France say they have proof it took place and, alongside the UK, are considering military retaliation.
Russia, which has military forces deployed in Syria in support of the government, has warned that Western air strikes risk starting a war.
During a press briefing on Friday, Mr Lavrov said he had “irrefutable evidence” that the attack was staged as part of a “Russophobic campaign” led by one country, which he did not name.
General Igor Konashenkov, a spokesman for Russia’s defence ministry, said: “We have… evidence that proves Britain was directly involved in organising this provocation.”
The UK’s envoy to the UN has called this a “grotesque, blatant lie”, Reuters news agency reports.
The White House says it is continuing to assess intelligence and talk to its allies about how to respond.
Meanwhile the UN’s secretary general has said the Middle East is “in chaos” and the Cold War is “back with a vengeance”.
Antonio Guterres was speaking to a special meeting of the UN Security Council, called by Russia.
He urged countries to “act responsibly in these dangerous circumstances”.
A delegation from the Organisation for the Prohibition of Chemical Weapons (OPCW) will arrive in Syria’s Eastern Ghouta region on Saturday. Few details are expected to be released about the movements of the chemical weapons inspectors, for safety reasons.
The suspected attack, denied by Syrian President Bashar al-Assad’s government, was carried out in the Eastern Ghouta town of Douma on Saturday, reportedly killing dozens of people.
Control over the town has since passed from rebels to the Syrian and Russian military authorities.
The Violations Documentation Center (VDC), a Syrian opposition network which records alleged violations of international law in Syria, said bodies were found foaming at the mouth, and with discoloured skin and cornea burns.
On Thursday, unnamed US officials told NBC News they had obtained blood and urine samples from victims which had tested positive for chlorine and a nerve agent.
The US ambassador to the UN, Nikki Haley, told the network: “We definitely have enough proof but now we just have to be thoughtful in our action.”
French President Emmanuel Macron also said he had “proof” that the Syrian government had attacked Douma with chemical weapons but did not give further details.
A UN report last year found the Syrian government responsible for a deadly chemical attack on the rebel-held town of Khan Sheikhoun, in which more than 80 people died.
That attack was followed by a US cruise missile attack on a Syrian airbase.
In the UK, cabinet ministers agreed it was “highly likely” the government of Syrian President Bashar al-Assad was responsible for the alleged attack and said the use of chemical weapons must not “go unchallenged”.
During a phone call on Thursday, UK Prime Minister Theresa May and US President Donald Trump had agreed to “keep working closely” on the issue, Mrs May’s office said.
The US president has said Russian President Vladimir Putin bears responsibility for the alleged attack in Douma because of his support for the Syrian government.
On Wednesday he warned Russia that missiles were “coming” but later tweeted that he had “never said when”. It “could be very soon or not so soon at all”, Mr Trump said.
Russia has described the reports of a chemical attack as a “provocation” designed to justify Western intervention.
Moscow’s UN ambassador Vassily Nebenzia accused Washington of putting international peace at risk.
“The immediate priority is to avert the danger of war,” he said on Thursday.
Senior Russian figures, including the head of the military, have warned that US missiles will be shot down and their launch sites targeted if Russian personnel come under threat.
On Friday, Deputy Prime Minister Arkady Dvorkovich criticised Mr Trump’s rhetoric.
“We cannot depend on the mood of someone on the other side of the ocean when he wakes up,” the Tass news agency quoted him as saying.
After six weeks of heavy fighting and an estimated 1,700 civilian deaths in the Eastern Ghouta, rebels are now leaving the area.
The final evacuations of about 4,000 remaining Islamist fighters and civilians were taking place on Friday, according to the UK-based Syrian Observatory for Human Rights monitoring organisation.