Angela Rayner could owe HMRC up to £3,500, a tax expert has warned.
The Deputy Labour leader has become embroiled in a row over whether or not she should have been liable to pay capital gains tax when she sold a house in 2015.
She sold her home in Stockport, which she bought through the right to buy scheme.
Rayner registered the former council house in Stockport as her main address on the electoral register, meaning she was not liable to pay capital gains tax on the £48,000 profit.
Angela Rayner could owe HMRC up to £3,500, a tax expert has warned
PA
But in 2010, weeks after she married Mark Rayner in 2010, she re-registered the births of her two youngest children at his address - a house just over a mile away.
Labour tax expert Dan Neidle has since argued that she could be liable to pay capital gains tax on the profits.
He said: "Now the story is out, it would be sensible for Ms Rayner to speak to a tax adviser and work out what her capital gains tax position in 2015 actually was.
“If it turns out she failed to pay a small amount of capital gains tax, I think most people would understand that as a mistake - but it's a mistake she can and should explain and correct."
Labour leader Sir Keir Starmer threw his weight behind Rayner, with a spokesperson saying he has "absolute confidence" in his deputy.
He said he is confident she has told the truth in her explanation and did not break electoral law.
Rayner denied any wrongdoing, saying she "lived there, paid the bills there and was registered to vote there" until she sold the property in 2015.
The revelation of Rayner's £48,000 profit on the sale of her home was first revealed by Lord Michael Ashcroft in his book. '
The Labour politician accused him of taking an "unhealthy interest" in her family life.
In a statement on social media, the deputy leader of the Labour Party said: "I bought my council house back in 2007. I owned my own home, lived there, paid the bills there and was registered to vote there, prior to selling the house in 2015. All before I was an MP…
"I've never been a ‘landlady’, owned a property portfolio or been a non-dom. As with the majority of ordinary people who sell their own homes, I was not liable for capital gains tax because it was my home and the only one I owned.
"My husband already owned his own home independently and I had an older child from a previous relationship.
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"Our son was born just 23 weeks into my pregnancy and spent eight months in intensive care. He is legally blind. We mutually decided to maintain our existing residences to reflect our circumstances.
"Every family is different but it worked for us and we brought up our boys in a caring environment, surrounded by love.
"A wide network of friends and family were also there to support us, including my brother. He’d served in Iraq and was a dab hand at DIY."
Rayner added: "For all the unhealthy interest taken in my family by Lord Ashcroft and his friends, there is no suggestion any rules have been broken. Just a constant stream of smears from the usual suspects."