The Prince of Wales' Duchy Originals food label is to go into partnership with supermarket chain Waitrose, letting it exclusively stock its range.
The royal firm says the move is about securing investment for the organic food company - and not because it is struggling in the recession.
Duchy Originals' profits, which fund The Prince's Charities foundation, have fallen sharply during the downturn.
Prince Charles said he hoped the tie-up would help expand his project.
He said: "When I think back to 19 years ago when we launched the first oaten biscuits there were large headlines saying 'Shop-soiled Royal'.
"So in an even more soiled capacity I am here today just to say this is a really proud moment because Waitrose is one of the great British stores."
Waitrose's managing director Mark Price said millions will be invested into developing the Duchy brand.
Duchy Originals made no profit in the last financial year and just £57,400 in the 12 months to March 2008, down from £1.5m in 2007.
However, Sir Michael Peat, the prince's principal private secretary, stressed the retailer was not "bailing out" the firm but helping it move to the "next level" by offering the reserves it needs to expand.
The partnership, which begins on 1 October, will give the high-end supermarket chain exclusive rights to originate, manufacture, distribute and sell Duchy products in the UK.
It aims to expand the range from 200 products to around 500 and will pay a royalty to Duchy Originals on all wholesale and retail sales.
The new partnership will see the head office of the prince's firm close and move into Waitrose buildings but will involve no redundancies among the 15 members of staff, the firm said.
Later the prince and the Duchess of Cornwall toured a Waitrose store in Belgravia, close to their Clarence House home in central London.
The duchess bought a selection of goods, which she planned to share with children at a community project in south London, and chatted to checkout worker Stella Amadasun.
At one point, the prince joked about the items she had bought which were produced by his company, saying: "It's like coals to Newcastle."