Health Secretary Jeremy Hunt is to be referred to the parliamentary standards watchdog over his failure properly to declare his involvement in the purchase of a series of luxury flats.
Mr Hunt has apologised following the disclosure that he failed to declare a business interest with both Companies House and the parliamentary register of MPs interests.
He said the omissions were the results of "honest administrative mistakes" and that he did not gain financially as a result.
However Labour said his actions represented a "serious breach" and that they would be referring him to the Parliamentary Commissioner for Standards.
Shadow cabinet office minister Jon Trickett said: "This is simply unacceptable and especially so given the Secretary of State's position at the heart of Theresa May's Government.
"Labour will today refer Jeremy Hunt to the standards commissioner to look into this serious breach. He should have had the decency to refer himself rather than sweep this under the carpet."
The move came after The Daily Telegraph reported that Mr Hunt failed to notify Companies House of his 50% interest in Mare Pond Properties Limited, something which took him six months to rectify.
He also did not inform the parliamentary register of members' interests of his share in the business within the 28-day time limit, according to the paper.
It is claimed Mr Hunt set up the company with his wife Lucia Guo to buy seven properties in the Ocean Village development in Southampton on February 7.
A spokeswoman for Mr Hunt said: "These were honest administrative mistakes which have already been rectified.
"Jeremy's accountant made an error in the Companies House filing, which was a genuine oversight.
"With respect to ministerial and parliamentary declarations, the Cabinet Office are clear that there has been no breach of the ministerial code.
"Jeremy declared the interest to them after the company was set up.
"They advised that as it was a shell company with no assets or value, it should only be registered when it became operational.
"As such, Jeremy presumed the same rules applied to Parliamentary declarations.
"Although there was no personal gain involved, Jeremy accepts these mistakes are his responsibility and has apologised to the parliamentary authorities."
A Downing Street spokesman said: "Jeremy has rightly apologised for an administrative oversight, and as the Cabinet Office have made clear there has been no breach of the ministerial code.
"We consider the matter closed."
The revelation comes as Mr Hunt is in Tokyo, Japan, where he is set to attend the Patient Safety global ministerial conference.