Eddie Izzard is to stage a surprise one-off gig at the Edinburgh Fringe tonight, 19 years after last performing at the festival.

He announced the show on Twitter, saying: "Very pleased to be doing my first stand up show at the Edinburgh Fringe in 19 years – this Monday at 10.45pm."

Izzard, now a Hollywood actor who has appeared in films such as Ocean's Twelve and Valkyrie, still packs theatres for his comedy tours and came to the attention of a whole new audience in 2009, completing 43 marathons in 51 days for Sport Relief.

He will be supported at his show tonight at the Assembly Hall by award-winning German comedian Michael Mittermeier and South African Trevor Noah.

Izzard said: "As an ex-street performer who used to perform on The Mound it is good to be back."

News of Izzard's gig emerged as it was revealed that this year's Fosters Edinburgh Comedy Awards 2012 are set to be the biggest ever, with 536 comedians in the running for the prestigious prizes.

The numbers of comedians performing new material at the Fringe has been going up consistently and this year there are 30 more performers than last year.

The winner of best comedy show receives £10,000, with £5000 being awarded to the best newcomer and the panel prize winner, but the main impact of the award is to turbo-charge performers' careers.

The shortlist is announced on August 22.

Nica Burns, director of the awards, said that while Fringe ticket sales have increased every year for the last three, it might be difficult during this "unique summer" to increase sales again.

She said: "Seeing what is happening in London, if this wonderful festival can be no more than 5% down on ticket sales this year, it will be an extraordinary achievement and fantastic accolade for this event."

She urged Edinburgh-bred Olympian Sir Chris Hoy to encourage visitors to come to the Scottish capital after the Games end.

Others, however, were more concerned about the impact the weather could have on the Fringe, after a damp opening Sunday with more rain to come.

In recent years, open-air eating and drinking facilities have been developed by all the major venues. This year, a section of George Street outside the Assembly Rooms between Hanover Street and Frederick Street has been cordoned off from traffic and pedestrianised, to make way for the Famous Spiegeltent and an al fresco bar area.

Tommy Sheppard, who runs The Stand and the revamped Assembly Rooms venue, said: "I just hope it's not as bad over the next four weeks as it was over the last four weeks or I'll be getting my P45."

Ticket sales, however, appear to be holding up in many venues in spite of the uncertain weather, with sales at The Stand on a par with last year's and at the Gilded Balloon up between 3% and 5% on the same period last year, with just over 36,000 tickets sold.

A spokeswoman for the Gilded Balloon said: "The shows have been doing well. Although the outdoor bars may be quieter, we have indoor venues so the weather hasn't had too much of a negative effect."