Inquiry follows Warsi's apology for failing to disclose business interest with relative who travelled with her on official visit

The political career of the cabinet minister Lady Warsi received another severe blow on Monday after she apologised to the prime minister and was then referred to the adviser on ministers' interests, for failing to properly declare her business dealings.

The co-chair of the Conservative party wrote to David Cameron admitting she failed to tell civil servants she and her relative Abid Hussain both owned a stake in a spice manufacturing firm when they visited Pakistan together on government business.

Cameron responded by referring her to Sir Alex Allan, his independent adviser on ministers' interests, who will consider whether Warsi has broken the ministerial code.

The prime minister's decision to refer Warsi to Allan, which could lead to a lengthy investigation, is a damaging blow for the forthright peer, who is already facing a Lords' inquiry and a possible police inquiry into her expenses. It has also sparked claims of hypocrisy from Labour; Cameron has refused to refer Jeremy Hunt, the culture secretary, over allegations that he broke the ministerial code on three occasions while overseeing the defunct BSkyB bid.

Warsi's letter to Cameron addresses her undeclared business links with Hussain, her husband's second cousin, who she says assisted the British High Commission with outreach events in Pakistan in July 2010. Their business relationship was disclosed in the Sunday Telegraph. Accounts for the firm, Rupert's Recipes, dating from February say she and Hussain are shareholders.

At first they both held a third of the company's shares, but Warsi's stake increased to 60%, according to the February accounts. According to the House of Lords' code of conduct members must register shareholdings in firms in which they hold a controlling interest or if they are valued at more than £50,000.

The code requires "unremunerated directorships" to be registered, as "certain non-financial interests may reasonably be thought to affect the way members of the House of Lords discharge their public duties".

In a letter to the prime minister, Warsi said she was sorry for the embarrassment caused by the undeclared relationship with Hussain, adding the fact that he was her husband's second cousin was "widely known", including to her private office and the British High Commission.

She said she did not realise the need to declare they had "a common business interest as minority shareholders in a small food company".

She wrote: "I sincerely regret that I did not consider the significance of this relationship with Mr Hussain when the arrangements for the visit were being made. In retrospect, I accept that I should have made officials aware of the business relationship between Mr Hussain and myself, and for this I am sorry.

"I regret that this failure may have caused embarrassment to the government."

The prime minister said he accepted her apology but was asking Allan to "consider the issues that have been raised with respect to the ministerial code and to provide advice to me as rapidly as possible". He wrote: "There are clearly some lessons for future handling."

Cameron has until now defended Warsi. In a cabinet dominated by white, privately educated men, she is seen as a breath of fresh air. But she has riled backbench MPs with her forthright views, and has been criticised for lacking experience.

Warsi's apology may not be enough to halt a party-wide whispering campaign against her. It follows a slew of allegations over her conduct over the last two weeks. She has been accused of claiming up to £2,000 for staying rent-free in the home of a Tory party donor.

The prime minister has faced criticism for refusing to refer Jeremy Hunt, the culture secretary, to Allan over the conduct of his special adviser Adam Smith in his dealings with Rupert Murdoch's empire.

Labour accused Hunt of misleading parliament on three occasions and said he broke the ministerial code, which states that ministers are responsible for the conduct of their special advisers – both reasons, they say, why he should be sacked.

Michael Dugher MP, Labour's shadow cabinet office minister, said it was right for Cameron to bring in Allan to consider Warsi's conduct and suggested other allegations against her should be considered.

"Did she break rules that peers should declare their business interests, particularly if they are the principal shareholders in a company? Why was it considered acceptable for a foreign government to pay for her visits to the Middle East? Did Baroness Warsi claim for expenses that she did not in fact incur?" he said.

"But David Cameron's actions in this case draw into sharp relief his refusal to hold a similar investigation into Jeremy Hunt … Cameron is bending over backwards to defend Jeremy Hunt because he knows that it is his own judgment, in appointing a man he knew to be biased to oversee the BSkyB bid, that is in question," Dugher said.