Vladimir Putin is to be confronted at the G20 summit of world leaders in St Petersburg this week with an array of western intelligence including damning new French evidence directly linking Syrian government forces with a massive and co-ordinated chemical attack on 21 August that led to hundreds of civilian deaths.

The Russian president will also be urged to show a new diplomatic flexibility and come closer to accepting that the Syrian leader, Bashar al-Assad, has to stand aside.

A nine-page declassified French intelligence report was released on Monday which claimed to show Assad forces had launched an attack on Damascus suburbs held by opposition units using a combination of conventional weapons and "the massive use of chemical weapons".

The report follows similar documents from British and American intelligence.

The Nato secretary-general, Anders Fogh Rasmussen also insisted that "personally I am convinced, not only that a chemical attack has taken place … but I am also convinced that the Syrian regime is responsible."

The French intelligence includes satellite imagery showing the attacks coming from government-controlled areas to the east and west of Damascus and targeting rebel-held zones. The report said Assad's forces had since bombed the areas to wipe out evidence.

"Unlike previous attacks that used small amounts of chemicals and were aimed at terrorising people, this attack was tactical and aimed at regaining territory," the report said.

Nearly 47 amateur video clips reportedly filmed on the morning of the attack and showing the impact on civilians had been authenticated by French military doctors, according to the intelligence. French evidence gave details of other suspected chemical attacks, in the towns of Saraqib and Jobar in April, which now appeared to have killed about 280 people, the report said.

The fresh information will also help Barack Obama in his uphill efforts to persuade Congress next week to back a punitive military strike against Assad.

Assad, in an interview with the French newspaper Le Figaro, again denied the use of chemical weapons. "Whoever makes accusations must provide proof," he said. "We have challenged the US and France to put forward a single piece of proof. Mr Obama and Mr Hollande have been incapable of doing so. The Middle East is a powder-keg, and today the spark is getting closer."

The Russian foreign minister, Sergey Lavrov, also rejected the western intelligence as absolutely unconvincing, saying that there was "nothing concrete, no geographical co-ordinates or details … and no proof the test was done by professionals".

He urged the west to lift the veil of secrecy over its allegations.

British sources said Putin, host of the G20, will face a concerted challenge at the summit on Thursday and Friday both to accept the intelligence, and propose a fresh diplomatic solution to the crisis by easing his support for the Assad regime.

Some senior Tories continued to put pressure on Downing Street to say it might be willing to hold a second Commons vote on whether Britain would in some circumstances participate in an attack on Syria in retaliation for the chemical attack.

Cameron unexpectedly lost the vote last week, but the former international development secretary Andrew Mitchell led calls for Britain to keep an open mind. "I believe taking legal and proportional action will be better than doing nothing – I think it will be more likely to bring this catastrophe to a close," he said.

"That's why I think it's very important in this rapidly moving situation that we don't rule anything out and it may be, for example, that after lengthy and careful consideration, Congress affirms its support for the President's plans and in the light of that our Parliament may well want to consider this matter further"

Advocates of a second vote were briefly given succour when the defence secretary, Philip Hammond, said there might be a second vote if there were a very significant change in circumstances.

But Labour sources poured cold water on a second vote, saying the government had ruled out military action. Apparently raising the bar on military action higher than last week, Labour sources said a second vote could only be countenanced if there was evidence that al-Qaida had got hold of significant stocks of chemical weapons, or there was a direct threat to national security.

The Labour motion last week was less specific about the trigger for action, and it was not clear if all shadow cabinet members were aware of this hardened stance.

Labour instead tried to refocus on diplomacy, with the shadow foreign secretary, Douglas Alexander, telling the Guardian: "An objective for the G20 summit in St Petersburg should be to establish a Syria contact group, like that which helped end the civil war in Lebanon. Unlike the Friends of Syria group, which was established as a forum for supporting the Syrian opposition, a Syria contact group could bring together those countries that are currently backing opposing sides in the Syrian conflict, of course including Russia, but also key sponsors within the region such as Iran and Saudi Arabia."

The contact group would work to bring the participants in the two-year civil war to talks in Geneva. Washington, with British backing, has opposed Iranian involvement in talks.

Writing in the Guardian, the senior Liberal Democrat peer Lady Williams has also called for Iran to be involved in the construction of a Syrian political settlement. She writes that the condemnation of the use of chemical weapons by the newly elected Iranian president, Hassan Rouhani, "deserves a constructive response. It is time to end the isolation of Iran."

Plans to stage a second round of peace talks on Syria in Geneva have been stalled since March, partly over the US veto on Iranian attendance, and partly due to disputes in the Syrian rebel forces over the composition of its delegation, and the presence of forces loyal to Assad. Cameron will not have been emboldened by the publication of three separate polls on Monday showing strong opposition to British involvement in military action. A BBC/ICM poll found 71% supporting the Commons decision to stop British involvement in military action with 67% saying that "the special relationship is not relevant to the modern age".