British IOC member Sir Craig Reedie believes Friday's vote for the 2016 Olympics host city will be one of the closest contests ever.

Chicago and Rio de Janeiro are viewed as the favourites ahead of Madrid and Tokyo, with the lobbying reaching a feverish intensity in Copenhagen ahead of the vote.

Reedie said: "This is a very, very close race between four outstanding bids, and because the bids are so good and because it is so close, the last few hours are going to be even more exciting than usual.''

US president Barack Obama is due to arrive on Friday but his wife Michelle flew in on Wednesday and immediately began the process of lobbying IOC members.

Other politicians and celebrities are also due including Oprah Winfrey for Chicago, Pele and Brazil president Lula for Rio, the new Japanese prime minister Yukio Hatoyama for Tokyo and King Juan Carlos of Spain backing Madrid.

Reedie added: "I think the IOC should be delighted and honoured that such an impressive group of political and royal figures should come to Copenhagen and ask for the reward of the Olympic Games.''

Meanwhile, the race has now been tarnished by allegations of dirty tricks.

Many of the 106 IOC members have been bombarded with emails purporting to be from Chicago residents saying they do not want the Games - and urging members to vote for Rio.

The boot has also been on the other foot with the finger of suspicion being pointed at Chicago backers for articles making claims about crime levels in Rio being sent to members.

Rio also lodged an official complaint with the IOC against Spanish Olympic Committee vice-president Jose Maria Odriozola who reportedly called the Brazilian city the "worst bid'' with security issues.

Most neutral IOC members accept that the Games are going west - ie to Rio or Chicago rather than east to Madrid or Tokyo.

The pendulum had gone towards the Brazilian city during the summer but in the last few weeks it has swung back towards the 'Windy City'.

Chicago had been plagued by controversies over financial guarantees and the US Olympic Committee announcing their own Olympic television channel without even discussing the project with the IOC.

Both those disputes have been settled, and instead it has been Rio, who had been enjoying considerable momentum, forced onto the back foot in recent weeks.

First, IOC president Jacques Rogge raised possible sponsorship conflicts with the 2014 World Cup, which is being staged in Brazil.

Rio have also been forced to insist that the cancellation of a World Cup swimming event in the city next month because of the lack of finance will not harm their 2016 bid.