The annual Norwood Green Village Day was due to take place on the green this Saturday (July 9).
But organisers have decided to change the location to nearby Norwood Hall Playing Field, after a large number of caravans appeared on the intended site.
The new site is behind Khalsa Primary School, in Norwood Green Road - a short walk from the green.
But organisers have now confirmed the fair will go ahead, albeit in a new location, on the same day from midday.
The free event will include a dog show, an Antiques Roadshow style valuation of small items and various charity stalls, among other entertainment.
First time event not held on Norwood Green in nearly 40 years
Mhairi Maclean, who is one of the organisers, said this would be the first year since the fair began in 1978 that it had not taken place on Norwood Green.
She told getwestlondon visitors could expect all the usual attractions at the new site, despite two stall holders having dropped out when they learned the green was occupied.
However, she said organisers feared the attendance might be down on last year, despite them spending extra money to publicise the change of venue, as the new site is harder to find.
"Everything's going ahead as planned, albeit at a different site and with a few more logistical problems," she said.
"All the usual stalls and activities will be there but there's a worry the event won't be as big and vibrant as it would otherwise have been, due to the change of venue."
'It's overcrowded and the grass will be getting damaged'
One local resident, who asked not to be named, said: "I don't mind them. They do their thing. But to be in the middle of a park by a massive roundabout isn't right.
"It's nice weather and people like to play cricket and football on the green but it's overcrowded with all the caravans and the grass will be getting damaged.
"I've been to fun fairs there with my little girl, which are great, and I'm worried they might not be able to take place now.
"I've been living here for 16 years and have never seen this before."
getwestlondon has reported how around six caravans were camped on Norwood Green near a children's playground last Monday (June 27) but that number had increased fourfold by Sunday (July 3), with the vehicles occupying much of the parkland.
The travellers are thought to have set up camp on Norwood Green after moving on from Ealing Common and Hanger Lane.
Ealing Council and the police have said they were engaging with people on the site and monitoring the situation, and tweeted on Monday to say a legal enforcement notice to move the camp had been served.