Angela Rayner's lawyers have declared they did not give her tax advice, claiming they were made to be "scapegoats".
Family company, Verrico & Associates, which handled the purchase of the Deputy Prime Minister's £800,000 flat in the seaside town of Hove in East Sussex, denied giving her tax advice.
The conveyancing firm based in Herne Bay in Kent, went on to say they had done "everything correctly and in good faith".
The claim is a major blow to Ms Rayner after she blamed "legal advice that I received" for her not paying the £40,000 stamp duty bill on the purchase of the seaside home.
Managing Director Joanna Verrico told The Telegraph it "acted for Ms Rayner when she purchased the flat in Hove".
"We did not and never have given tax or trust advice," she added.
"It's something we always refer our clients to an accountant or tax expert for.
“The stamp duty for the Hove flat was calculated using HMRC’s own online calculator, based on the figures and the information provided by Ms Rayner."
The firm confirmed it acted for the Deputy Prime Minister
|GETTY
Ms Verrico said: "That's what we used, and it told us we had to pay £30,000 based on the information provided to us".
"We believe that we did everything correctly and in good faith," she said.
"Everything was exactly as it should be."
Ms Verrico added that "we probably are being made scapegoats for all this".
Angela Rayner admitted to a stamp duty underpayment
| GETTY"And I have got the arrows stuck in my back to show it," Ms Verrico said.
"We are not an inexperienced firm, but we're not qualified to give advice on trust and tax matters and we advise clients to seek expert advice on these."
A Council for Licensed Conveyancers spokesman said: "We have asked them for a full account of events".
"They are not licensed to provide tax advice," the spokesman said.
"What they are licensed to do is to provide conveyancing advice, and they are regulated to the highest standards."
Giving her take on the latest developments, Conservative Party Leader Kemi Badenoch said "this is yet more damning evidence that Angela Rayner has not been honest with the British public".
"From the start, we've had nothing but excuses, deflections and lies," she said.
"Enough is enough."
Ms Badenoch asked "how many final straws" can Ms Rayner have.
"She must resign or Keir Starmer must finally find the backbone to sack her," she added.
Ms Rayner admitted on Wednesday to underpaying £40,000 of stamp duty on the Hove flat, saying she had relied on "legal advice".
The Prime Minister's independent adviser on ministers' interests, Sir Laurie Magnus, is investigating the Deputy Prime Minister's claim and is expected to come back as soon as Friday.
Sir Keir Starmer said on Thursday he was willing to sack Ms Rayner if she was found to have broken the ministerial code.