They were pictured chatting about the benefits of organic food in a food hall resplendent with purple calabash tomatoes, butternut chutney and smoked salmon from the Shetland Islands packaged in wrappers adorned with hearts.

But the smiling photo-opportunity is now exercising Cameron’s political opponents, who accuse him of endorsing companies and products that his friends and his own family are associated with.

Carole’s husband is Sir Anthony Bamford, head of the JCB digger dynasty and donor of £1m to the Tories before last year’s general election.

Since the farm shop visit in March, Cameron has registered his gratitude in the House of Commons register of members’ interests for the free use of a Sikorsky S-76 helicopter with tinted windows and walnut-veneer drinks cabinet belonging to Bamford.

Labour MPs, stung by the revelations in The Sunday Times that their party has been involved in a “cash for honours” scandal to secure donations, are now demanding more transparency in the links between Cameron’s politics and his business pals. The Liberal Democrats have gone further and accused him of “product placement”.

Cameron appears undeterred. Next month he is off to India on an official visit during which he will find time to open JCB’s new £25m factory in Pune. The company will provide him and his team with at least one internal flight. Bollywood actresses and Narain Karthikeyan, the Indian Formula One racing driver and JCB’s “brand ambassador” on the subcontinent, are among those expected to be there.

Cameron last week hit out at any suggestion of a conflict of interest in his support for particular companies. He said he was “careful” about the firms he mentioned in his speeches. But a detailed study of the companies he has praised for their social responsibility shows that six have been clients of Good Business, a consultancy set up by Steve Hilton, his £276,000-a-year chief strategist. Hilton has worked full time for the Tories since the beginning of the year.

The companies include Asda, BP, British Sky Broadcasting, British Telecommunications, Coca-Cola, and Nike.

In April, as part of a speech in Norway on climate change, Cameron cited the environmental campaign undertaken by BP as evidence that “focusing on energy efficiency can add substantially to the bottom line”.

He has also suggested that Coca-Cola has “a better distribution network in sub-Saharan Africa than any aid agency”.

It is not the first time Cameron, who used to be a public relations executive for Carlton television, has been caught out helping his friends and family.

When he was interviewed by GQ magazine, he heaped praise on the “many extremely fine products” made by Smythson, an upmarket stationer and leather goods company with a shop in Bond Street, London. His wife Samantha is the firm’s creative director.

On Desert Island Discs, the BBC radio programme, he plugged Hugh Fearnley-Whittingstall’s River Cottage Cookbook as “brilliant”. The chef is a fellow old Etonian and an Oxford University pal whose cousin Sarah is married to Cameron’s brother Alexander.

He also chose as his luxury item a case of Isle of Jura malt whisky, made by Whyte & Mackay, a firm co-owned by Robert Tchenguiz, a property tycoon and Tory donor.

Jim Dowd, the Labour MP for Lewisham West who is monitoring Cameron’s links to his financial backers, said: “I find his business interests rather strange. The Bamfords have stopped contributing directly to the Tory party, although free helicopter and plane rides fall into the bracket of payments in kind. Opening the JCB factory in India may be an attempt to rekindle Sir Anthony’s enthusiasm for the Conservative party.”

Cameron said last week: “I try to be careful in these matters but I talk about corporate responsibility. I think it’s important. I mention dozens of companies and I try to be careful of the ones that I mention.”

His spokesman said: “David Cameron is going to India to highlight how the country is rapidly emerging in the global economy. It just happens that while he is out there JCB are also opening a new factory and it’s a British company so why not go there? If there is any gift in kind it will be declared in the register of members’ interests.”

Additional reporting: James Grande; Damandeep Singh in Delhi

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