The historic estate will tomorrow open its gates to the public, who are expected to descend in their hundreds clutching family heirlooms they want to be valued.

Preparations have been going on for days at the Althorp estate and the Earl said he is particularly excited at the prospect.

He said: "This is a very big year for my family and the house, as it is the 500th anniversary of our living there.

"I suppose it's therefore suitable, as I celebrate my family's history of collecting art and furniture, to host a programme like the Antiques Roadshow at Althorp, which often looks at objects that have also been handed down by previous generations.

"Looking after Althorp is a responsibility I take very seriously, but it does have many fun benefits – welcoming the BBC to film such a well-loved programme there is definitely one of them."

Many of the public will also be hoping for a glimpse of new presenter Fiona Bruce who will also be on site.

The ever-popular Sunday evening programme will be filming for its 31st series in the grounds of Althorp, where the gates open at 9.30am and close at 4.30pm.

Series editor Simon Shaw said: "The team are all looking forward to visiting Althorp. It's always exciting to see what will come to light on the day. We regularly see between 1,500 and 2,000 visitors on the day. Despite the high turnout everyone will get to see an expert."

The antiques experts will be on hand to offer free advice and valuations to visitors, who are expected to bring family heirlooms, household treasures and car boot bargains with the hope of a life-changing estimate.

Earl Spencer said: "I hope everyone who comes has a memorable day – and, who knows, maybe some Northamptonshire family will be pleasantly surprised to find that an old heirloom they knew little about, turns out to be a priceless treasure? I hope so."