A senior figure at the Bank of England has admitted breaching the bank's own guidelines covering potential conflicts of interest - rules she even helped put together.
The disclosure by Charlotte Hogg, who became deputy governor for markets and banking this month, prompted one leading MP to demand her resignation.
It is seen as a conflict of interest because she sits on the Bank's Prudential Regulation Committee (PRC), which helps regulate the financial sector.
She (Munich: SOQ.MU - news) made the admission in a letter to the Treasury Select Committee, having told MPs (BSE: MPSLTD.BO - news) during an evidence session last week about her brother's role, adding that she had always declared areas of conflicts of interest and was compliant with all of the Bank's codes of conduct because she had helped write them.
The discrepancy emerged afterwards when she and Bank officials checked her disclosure and found no entry concerning her brother Quintin's job.
Labour MP John Mann, a member of the committee, said: "It is simply incredible that such a senior person at the Bank of England has behaved in this manner.
"This is simply a question about standards in public life and in this regard she has failed and must resign."
Andrew Tyrie, chairman of the committee, said some of the evidence it heard "has been a surprise to us" and it would "offer a view after a period of reflection".
In her letter to MPs, Ms Hogg wrote: "As Barclays Bank plc is regulated by the Prudential Regulation Authority, under the Bank's internal code of conduct and personal relationships policy, I should have formally declared my brother's role when I first joined the Bank.
"I did not do so and I take full responsibility for this oversight. I have now added a full record of my brother's role in the Bank's HR systems.
"Regrettably, my oversight means that my oral evidence to the committee in this respect was not accurate. I write now to correct that evidence at the earliest opportunity and to place on record my sincere apologies to the committee."
On Tuesday, the MPs were told by the chairman of the Court of the Bank of England, Anthony Habgood, that it was an "honest mistake" though she was "clearly going to sit on the newly formed PRC and such a relationship should be declared for obvious reasons".
"It's a very serious breach," said Mr Habgood.
His deputy, Bradley Fried, added that senior managers would be "grumpy about it" but it would not lead to a written warning which would be notifiable to the regulator, or grounds for dismissal.