David Cameron's new environment minister was thrust into the spotlight yesterday after questions were raised over her close links to agriculture lobbying.

The Sunlight Centre, an anti-lobbying group which campaigns for more transparency in politics, called for more scrutiny of Caroline Spelman's former directorship of the food and biotechnology lobbying company Spelman, Cormack and Associates. Spelman, 52, set up the firm with her husband, Mark Spelman, in 1989.

Although she resigned as a company director last year, the firm remains in the hands of her husband and still has her maiden name – Cormack – on the company's letterhead.

The Sunlight Centre said it had forwarded a letter about a possible conflict of interest in Spelman's appointment to the permanent secretary at the Department for Environment, Food and Rural Affairs – the department Spelman now heads.

"Her husband, Mark Spelman, remains a director of the company still trading under the secretary of state's name in sectors closely related to issues for which she is responsible," the letter said. "Given that the company is still using her maiden name to trade, a name by which she would have been known when active in farming politics in the 1980s, this is clearly of public interest."

According to Companies House, the company address was also transferred from Spelman's constituency home, where it was registered until May last year – around the same time she also transferred over company shares to her husband and resigned her directorship.

Spelman has made no secret of her agri-lobbying past. She served as sugar beet commodity secretary for the National Farmers Union in the early 1980s before becoming an MP and was deputy director of the International Confederation of European Beet Growers in Paris.

She faces questions over how suitable she is to hold a portfolio which includes responsibilty for GM crops and farming subsidies for the department her firm spent a decade lobbying.

The Sunlight Centre said it had called for more transparency over which agri-businesses her former company had had dealings with, and urged clarification over her role in negotiations involving the sugar beet industry. Spelman's agri-lobbying links were also highlighted by the rightwing Guido Fawkes blog, run by the political-agitator Paul Staines.

A Defra spokesman said: "Caroline Spelman relinquished the unpaid directorship of Spelman, Cormack and Associates in June 2009. The secretary of state and Defra's permanent secretary will be ensuring in the usual way that the secretary of state's private interests are declared and handled in line with the ministerial code." He added that the company had made no revenue in the past five years.

Although there is no suggestion that Spelman has acted untowardly, the issue raises uncomfortable questions for the new prime minister, who said in February that secret corporate lobbying was "the next big scandal waiting to happen".

Cameron promised tougher controls, proposing a two-year hiatus on lobbying for ministers who had left government. "It's an issue that crosses party lines and has tainted our politics for too long, an issue that exposes the far-too-cosy relationship between politics, government, business and money," he said. It is not clear how Cameron will deal with the issue around Spelman, who had long been a member of the Tory shadow cabinet, holding positions including communities and local government secretary.

In March, the former cabinet ministers Stephen Byers and Geoff Hoon, along with a number of others, were caught up in a sting operation earlier in the year, apparently offering lobbying services.

It is not the first time that Spelman has come under scrutiny. In 2008 she was criticised for the so-called "Nannygate" affair in which misused her parliamentary staffing allowance to pay her children's nanny, against parliamentary rules, and was ordered to repay the amount.