Sir Craig said following the Foundation Board meeting that the document was the "most encouraging" piece of correspondence they had received from Russia amid the country's doping scandal.

International Olympic Committee (IOC) members and sports movement representatives had claimed the letter constituted an acceptance of the McLaren Report, one of the two remaining reinstatement criteria RUSADA has yet to fill along with allowing access to the Moscow Laboratory.

Sir Craig, himself an IOC member, did not endorse that view but claimed he was hopeful it would be "a game-changer" in the deadlock between WADA and Russia.

The letter, sent to the Executive Committee prior to their heated meeting yesterday, will be analysed by the Compliance Review Committee (CRC) at a meeting on June 14.

CRC chairman Jonathan Taylor promised the CRC would report back as promptly as possible.

The letter has raised the chances of RUSADA being declared compliant quicker than many accepted as Russian officials had previously insisted they would never accept the findings in the McLaren Report, which says that an institutional doping scheme was in operation at events including the Sochi 2014 Winter Olympic Games.

The contents of the letter remain unknown but it was seemingly strong enough to spark IOC members and sports movement representatives to call for a provisional reinstatement of RUSADA, first declared non-compliant in November 2015, during the Executive Committee meeting.

"I hope it is a game-changer," said Sir Craig.

"We've been exchanging correspondence now for the best part of a year-and-a-half and there have been very many differentiations of the words, this one is probably the most encouraging one that we have had.

"My first look at it is that they are getting closer to acknowledging that there was an error.

"Someone said to me it is only semantics but semantics matter in this game – they matter in Moscow and they matter within this organisation and let’s hope I am right."

Sir Craig told insidethegames that was hopeful a direct contact could be established with the Investigative Committee of Russia to allow WADA access to the Moscow Laboratory and the samples and data stored there.

RUSADA director general Yury Ganus called on the Committee to open the Laboratory earlier this week, marking a significant shift in rhetoric after he previously refused to criticise the group.

In response, Chairman of the State Duma Committee for Physical Culture, Sport, Tourism and Youth Affairs Mikhail Degtyarev said they had not yet done so as they do not trust WADA following Russian accusations that the organisation has a political agenda against the country.

"We can say that there is a crisis of confidence, but it is not connected with the position of Russia, but with the position of WADA," he was quoted as saying by Sport Express.

"Everyone should understand: trust is a two-way road."

The 77-year-old Scot, whose term as WADA President comes to an end next year, declined to put a timeframe on RUSADA's potential reinstatement.

But he acknowledged comments from IOC member Patrick Baumann, who said the sports movement was concerned the the two outstanding requirements may never be fulfilled and RUSADA's non-compliance would "drag on and on".