Tickets for the event at Ampthill Park sold out many weeks ago with around 6,000 people expected to attend - a new record.
But organisers fear ticketless fans will turn up anyway, hoping to get in.
“They will be disappointed,” says AmpRocks chairman, Mark Tiana.
“We cannot stress this enough: If you do not have a ticket, you will not get in, so please do not turn up without one. It is as simple as that.
“Tickets first went on sale nearly five months ago and we urged people then to buy them up because demand has been phenomenal for the past three years.”
Organisers reached agreement with authorities to increase this year’s capacity by around 1,000 after being swamped by demand for tickets.
Tomorrow’s gig - headed up by Scottish rockers The Fratellis, with The Pigeon Detectives supporting - is the third AmpRocks on the trot to sell out.
“We would love to make more tickets available but we have a fantastic safety record and we have hit our limit,” said Mark.
“We have a brilliant security team in place, a brilliant line-up on stage and we are expecting another amazing night in a fantastic venue.
“What we do advise people to do is to keep tabs on social media - you may just find someone who has a spare ticket or two. But there will be no tickets at the gate - sorry!”
The following night (Saturday July 2) the park hosts Ampthill Proms - tickets ARE available on the gate.
On Sunday, the festival weekend winds up with a free Gala Day at Ampthill Park. BBC children’s entertainers Dick ‘n’ Dom headline with Queen tribute act Mercury closing the festival with a free gig.