Paralympic marathon champion Kurt Fearnley is "gutted" that he will not be able to defend his London Marathon men's wheelchair title on Sunday.

The Australian who broke the course record last year has been forced to withdraw from this year's race because of the flight disruption caused by the cloud of volcanic ash hanging over northern Europe. Fearnley was planning to travel directly to London from Australia but has had to accept that he won't be able to get to Britain in time to compete.

"I am feeling pretty gutted at not being able to come to London but there is nothing I can do," said Fearnley, who defeated Britain's David Weir last year to win the London crown for the first time.

"I have worked really hard over the last eight weeks to be ready for this race and can't believe it's not going to happen. The London Marathon organisers have done everything they can to get me there but it's just not possible. I will really miss being there but good luck to everyone else."

Fellow Australian Christie Dawes has been forced to pull out of the women's race for the same reason. Dawes, who was fifth in 2009, was hoping to challenge for a podium place this year.

Virgin London Marathon Race Director Dave Bedford said: "We are very disappointed to lose our reigning champion. Kurt has raced here for the three years and we were looking forward to welcoming him back as champion and course record holder. Unfortunately, circumstances have conspired against us this time.

"However, despite losing Kurt and Christie both the men's and women's wheelchair fields remain top quality and we fully expect two thrilling races on Sunday."

Fearnley's absence leaves Weir as favourite to win his fifth London Marathon title. The Briton, who was London champion three years in a row between 2006 and 2008, was devastated to lose his crown last year. He will still face a stern test, however, not least from South African Ernst Van Dyk, the fastest man in history, who won an incredible ninth Boston Marathon title on Monday.

Van Dyk will be travelling from Boston to London via Spain along with a number of other elite wheelchair competitors, including Wakako Tsuchida, the Japanese athlete who won the women's race in Boston for the fourth consecutive year this week, and London's reigning women's champion Amanda McGrory who was third.