Labour frontbencher Jack Dromey made a "serious" failure to declare his work for the Unite union when he became an MP, according to the Commons watchdog the Standards and Privileges committee.
However standards commissioner John Lydon said any breaches were "not intentional", concluding: "We acknowledge Dromey's co-operation with the inquiry and his immediate and repeated apology."
The committee acknowledged the Labour MP was a "new and inexperienced member" when he failed to declare money he got from work with the union Unite.
The MP will apologise in parliament and has updated registers to show he received £30,000 after leaving his position as Deputy General Secretary of Unite to become a member of parliament and a further £27,867 payment.
In a letter to Lydon, Dromey said he accepted he was at fault and "took steps to remedy the situation" as soon as possible.
The light slap on the wrist will come as a relief to the shadow housing minister, who was investigated after a complaint by Tory MP Andrew Bridgen.
In his original letter Bridgen claimed: "As a former Labour party treasurer and with a wife, Harriet Harman, who has been an MP since 1982 Mr Dromey would have been fully aware of the need to register payments he was receiving from Unite."
Making his personal statement to the Commons, Dromey said: "I would like to make an apology to the House.
"A report has been published by the Standards and Privileges Committee following an investigation by the Parliamentary Commissioner for Standards.
"I failed to update in time my initial registration in respect of payments received from my previous employer during the months of June to October 2010. This I have now done.
"I also failed to declare an interest in speaking in two debates on June 16, 2010 and September 16, 2010.
"Notwithstanding that the commissioner and the committee noted that the breaches were unintentional, I want to apologise unreservedly to the House and I will in future fully abide by the rules of the House."
Commenting Bridgen said: “Once again, Labour have been found guilty of putting the vested interest of the trade unions ahead of the interest of the British people.
“Ed Miliband can talk all he likes about taking on the unions, but unless he publicly reprimands his Shadow Minister’s misconduct, people will see him for what he is – weak and in the pockets of his union paymasters.”