South Africa remains the strong favourite to be awarded the 2023 Rugby World Cup on Wednesday but there is growing concern that a failure to fully engage in behind-the-scenes lobbying may have opened the door for Ireland or France.

After two weeks of rancour and recrimination from the Irish and French following World Rugby’s evaluation report and subsequent recommendation in favour of South Africa, the council of the sport’s governing body will make its decision in a secret ballot in London – 39 votes are cast, a simple majority of 20 is required for victory.

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World Rugby is anxious that its council members endorse its recommendation – designed to eliminate the kind of horse-trading that has been rife in the past – but rather than admitting defeat, France and Ireland both stepped on to the offensive, leading to claim and counter-claim over the accuracy of the 139-page report.

South Africa’s best chance of victory is to reach the required 20 votes in the first round. If it does not obtain the necessary majority, however, and is pitted in a two-horse race with Ireland, there are growing fears of defeat. Insiders believe that the South Africa Rugby Union chief executive, Jurie Roux, was wrong to urge France and Ireland to withdraw from the race following World Rugby’s recommendation. Their concern is that Roux was perceived as arrogant and gave France and Ireland carte blanche to go on to the attack.

The chief executive of the Ireland union, Phillip Browne, went as far as to write a letter to World Rugby outlining his grievances.

On Friday it was greeted with a terse response from World Rugby, who consider the matter closed, but one insider has predicted Ireland will gain at least 19 votes in a two-horse race with South Africa due to extensive lobbying with those such as the USA and Germany – both of whom are looking to join the Pro14, which has its headquarters in Dublin.

It is understood Roux has recently been canvassing for South American support but there are fears that he has not done enough lobbying, instead placing too much faith in a perceived obligation among council members to follow the recommendation.

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England’s reported decision to back Ireland is not surprising after Theresa May gave her official backing in September but it will only worsen South Africa’s fears. The RFU maintains that it was always going to carry out its own due diligence after England staged the World Cup in 2015, but if its three votes do go to Ireland, it can hardly be seen as an endorsement of World Rugby’s recommendation considering the governing body’s chairman, Bill Beaumont, is a former England captain and RFU chairman.

It is the first time that World Rugby has commissioned and made public a report such as this and there is therefore no precedent as to whether council members rubber-stamp South Africa as 2023 host, or vote according to other criteria. It will be of great embarrassment to World Rugby should Ireland or France prevail, however, considering this process was adopted in the interests of transparency.

France has thrown caution to the wind in its attempts to invalidate World Rugby’s report, described by its federation’s president, Bernard Laporte, as “nonsense”. Laporte has been leading the last-minute charm offensive in recent days and France’s greatest hope is its financial strength. The bid leader, Claude Atcher, promised in September that a France victory would prevent “the death of world rugby” with its financial muscle, and pledged to stop the player drain from the southern hemisphere to the Top 14. Both New Zealand and Australia, however, are expected to back South Africa.