Travellers have been accused of littering an RSPCA centre's showground with debris and human excrement in the run-up to the charity's biggest fund-raising event of the year.

Television's "Mr Nasty", Simon Cowell, the charity's patron, is billed as special guest at the open day on Sunday but organisers say they will have a race against time to clean up the mess.

Organisers at the RSPCA, in Braypool Lane, Patcham, Brighton, are determined the event will go ahead, having spent six months preparing for the big day.

It is expected to attract more than 2,000 visitors.

Organisers feared they would have to cancel the event when travellers arrived on the field in June.

They were assured by Brighton and Hove City Council the travellers would be moved on within weeks but staff and volunteers have watched helplessly as more travellers arrived.

The council plans to evict the travellers today and has already arranged for contractors to clear up the mess in time for the event.

But RSPCA staff feel they have had little support from the council and believe action should have been taken sooner.

Chairman Frances Lindsay-Hills said: "There is toilet paper and excrement everywhere and it's just the most appalling mess.

"We've had no support from the travellers liaison unit or the council. The police have been helpful but they can only act on the council's instructions.

"We're still planning to go ahead on Sunday - we've got to. We've people coming from all over the country.

"We've got Simon Cowell coming and other celebrities. We've arranged a huge auction with holidays and crates of champagne as prizes.

"It's our biggest event of the year so we can't cancel it."

The travellers' presence on the field has meant volunteers have had to walk their 90 dogs elsewhere.

The council arranged a court date for last Monday after 19 vehicles arrived on June 24.

When a further 36 vehicles moved on to the site last Wednesday night the council arranged for a Section 61, ordering them to move off last Thursday.

But a council spokeswoman said the London bombings meant the police had to be on stand-by to help the capital instead, resulting in the action being delayed until the court case on Monday.

She said: "We have done everything we can. The travellers have to move on today or they will be evicted.

"Contractors will be on site after the eviction to clean the field and ensure it is ready for the RSPCA open day and the many other people who use the land for sport and recreation."

Mr Cowell said: "I'm really looking forward to Sunday's event and I hope it all goes ahead because it's so important for the RSPCA and for the people who look after the centre throughout the year. It was so good last year and, hopefully, it will be even more of a success this time.

"It's been a great anxiety for the staff with the travellers being there but hopefully they will be gone by the end of today and we'll be going ahead for the open day on Sunday."

Emma Nuttall, unit manager of the national organisation Friends Families and Travellers, said the lack of official sites provided by the council meant travellers had no option but to stay on unofficial sites such as Braypool field.

She said the only official site at Horsdean was closed to new people because it was being refurbished.