The parliamentary standards watchdog has cleared Tessa Jowell of failing to declare a shareholding involving her estranged husband David Mills.
Ms Jowell said she had 'never heard' of the company
Sir Philip Mawer said MPs did not have to register shareholdings in which a spouse alone had a beneficial interest.
Tory MP Nigel Evans raised the matter amid claims the culture secretary's husband made £67,000 from the shares.
Meanwhile, the Cabinet Office said an inquiry into Ms Jowell's financial dealings would not be reopened.
The Cabinet Office made it clear that Cabinet Secretary Sir Gus O'Donnell would not be mounting a fresh inquiry into similar claims by Mr Evans that Ms Jowell broke the ministerial code.
At the weekend, a newspaper alleged that Mr Mills had made the £67,000 profit after buying shares in the Old Monk Company pub chain in 1998, when Ms Jowell was a public health minister.
This led Mr Evans to claim there had been a conflict of interest involving Ms Jowell.
On Monday, the culture secretary said she had "never heard of this company or the transactions until this weekend".
She added: "I understand the shares were never owned by my husband."
In a letter from Sir Philip to Mr Evans on Wednesday, the parliamentary standards commissioner says: "In short, even if (and this is, I understand, denied by him) Mr Mills was the beneficial owner of the Old Monk Company shares, Ms Jowell was not obliged by the rules of the House to register them."
On Monday, Sir Philip told Ms Jowell she did not need to give any more details about her husband's financial dealings.
Mr Mills and Ms Jowell announced they were separating at the weekend.
Mr Mills' lawyer blamed this on the furore over a £344,000 payment.
Italian prosecutors have been examining claims the money was paid to Mr Mills in return for helpful testimony in a corruption probe concerning Italian prime minister Silvio Berlusconi in 1997.
Last week, Ms Jowell was cleared by Tony Blair of breaching the ministers' code of conduct - because a report said her husband had not told her about the gift.
Both Mr Mills and Ms Jowell have denied any wrongdoing. Mr Mills has said the money came from another client.