The former chancellor initially failed to declare that he was setting up a Northern Powerhouse think-tank despite having launched the same policy as Chancellor.
The Advisory Committee on Business Appointments (ACOBA) was only told of Mr Osborne’s plans after they appeared in the press.
Former ministers are barred from lobbying the Government for two years after they leave office and “must seek advice from Acoba about any appointments or employment they wish to take up within two years of leaving office”.
The body however has no actual powers to sanction Mr Osborne for the late declaration.
In a letter to Mr Osborne, Acoba said: “The Committee would also remind you that advice should be sought on all appointments, paid or unpaid, before they are taken up or announced.”
Lib Dem leader Tim Farron said: “The Chancellor has been rapped over the knuckles for not following due process – having been in the government for the last few years he should know these rules and abide by them.
“It’s not good enough for him to bend the ministerial code simply to try and protect his failing Northern Powerhouse project.”
Mr Osborne’s office has yet to comment on the matter.