Angela Rayner used £160,000 from a trust set up to care for her disabled son to buy a £800,000 seafront second home.
The Deputy Prime Minister sold a 25% share of her house in Ashton-under-Lyne to the trust for £162,500.
She revealed on Wednesday that her son, who was born prematurely, had received an award in 2020 and that a trust was then set up to look after his interests.
It is understood that the payout followed an 11-year legal battle waged between the Rayners and the hospital where her son was born.
Sources told the Telegraph that the NHS had paid compensation following difficulties during his birth and subsequent care in 2008.
Final payments can take years to be made because of the lengthy process for assessing long-term damage.
It is unclear how much money was paid out as part of the damages claim but it was sufficient for the trust to buy Ms Rayner’s share at the end of January this year.
She then used the money as a deposit on the flat in Hove that she purchased in May this year, on which she now has a £650,000 mortgage
Ms Rayner, who is also the Housing Secretary, said she had received inaccurate legal advice that led her to underpay tax when buying the flat.
She has been under pressure after media reports claimed she saved £40,000 in stamp duty on the property because she removed her name from the deeds of a family home in her Ashton-under-Lyne constituency, classifying the Hove flat as her only property despite still spending time at the family house.
In a statement on Wednesday, she said she had taken legal advice when she bought the south coast flat, which suggested she was “liable to pay standard stamp duty”, but had then sought “further advice from a leading tax counsel” after headlines about the arrangement.
She learned that the initial advice had been inaccurate and she was liable to pay additional stamp duty.
Tax experts said the Hove property could not be treated as her only residence because of the nature of the trust.
Ms Rayner spoke out hours after a court order was lifted which had prevented her from disclosing information about her property arrangements.
Tory leader Kemi Badenoch called for the Deputy Prime Minister to be sacked amid accusations of hypocrisy as ministers may be considering property tax rises in the autumn Budget.
In the Commons, Sir Keir said: “She has explained her personal circumstances in detail. She’s gone over and above in setting out the details including, Mr Speaker, yesterday afternoon, asking a court to lift a confidentiality order in relation to her own son.
“I know from speaking at length to the Deputy Prime Minister just how difficult that decision was for her. But she did it to ensure that all information is in the public domain.
“She has now referred herself to the independent adviser. That is the right thing to do.
“But I can be clear, I am very proud to sit alongside a Deputy Prime Minister who is building 1.5 million homes, who is bringing the biggest upgrade to workers’ rights for generations, and who has come from a working-class background to become Deputy Prime Minister.”