With pairs of tickets for Saturday’s rugby cup final changing hands for more than £3,000, one person can be assured of a free seat: Gordon Brown.

The Prime Minister has accepted an invitation to the match by the Rugby Football Union (RFU), Downing Street confirmed today.

Mr Brown joked at his No 10 press conference last week that he had no trouble switching his allegiance to England once Scotland had been knocked out of the tournament.

He said: “The one barrier to my support for England in the World Cup Final has been removed by the unfortunate defeat of Scotland.”

Now the Prime Minister must keep his fingers crossed that the European Union summit he is attending in Lisbon does not over-run. It is due to finish on Friday.

While most of the tickets for Saturday’s confrontation between South Africa and England have already been sold, some are kept back until the week before the final. The RFU will announce how the few remaining tickets will be distributed at 5pm today.

Tickets for the final are already fetching thousands of pounds, according to the chief executive of an online ticket exchange.

Eric Baker, founder and chief executive of viagogo, said: “For the final, there are still some bargains available where people are selling for as little as £500 and there are some other seats in really prime locations that are going for north of £3,000.”

He added that the event was “the hottest ticket we have seen in the last two years, quite frankly. It is arguably a more coveted ticket than for the Led Zeppelin concert.”

Bidding for tickets on the online auction site eBay ranged from up £2,000 a pair to one seller who offered two match and plane tickets for £9,999.

Most fans will have to content themselves with viewing the match on television. For those who prefer a big screen, the RFU plans to screen the final at Twickenham stadium. The Twickenham Experience is selling a package that includes complimentary drinks and food for £67.50 at www.twickenhamexperience.com.

A spokeswoman for the RFU said it was too soon to say where other big screenings of the match would take place.