The conveyancing firm that Angela Rayner used to complete her purchase of an £800,000 flat on the south coast has said it did not offer her tax advice and completed her stamp duty return based only on information she provided.

Joanna Verrico, the head of a small, family-run firm in Kent, said on Thursday it had not provided any advice to the deputy prime minister on how much stamp duty to pay.

The advice given to Rayner is at the heart of questions over why she underpaid her tax by as much as £40,000, and is likely to form a core piece of a report from the prime minister’s standards adviser, Laurie Magnus, which is due to be submitted on Friday.

Keir Starmer refused to say whether he would sack Rayner if she was found to have broken the ministerial code when asked repeatedly on Thursday. He said: “My experience is [Magnus] will be comprehensive in the report that he gives me. He will be quick, and that’s what I’m expecting. And so I want to let that process take its course.”

Verrico, whom the Guardian revealed on Thursday was the conveyancer for the purchase, said in a statement: “I confirm my firm does not deal with trusts or offer tax advice. The stamp duty land tax was calculated using the HMRC calculator and was strictly based on the facts and information provided to us.”

She added: “We believe we did everything correctly and in good faith.”

The statement adds to the pressure on Rayner, who says she was given legal advice before buying the flat in May that she only needed to pay the lower rate of stamp duty because she did not own any other property.

Supporters say the deputy prime minister believed she did not need to declare an interest in her family house in Greater Manchester because she had sold her stake in it to a trust in the name of her disabled son. However, the terms of that trust meant she retained an interest in the property for tax purposes.

Rayner’s allies say she did not initially realise the tax implications of her son’s trust, even though she is understood to have consulted two experts on trust law before completing the purchase.

After details of the purchase were published in the press, Rayner then sought separate advice from Jonathan Peacock, an experienced barrister specialising in tax. He gave her his draft advice on Monday, even as the prime minister continued to defend her, and a final opinion on Wednesday, after which she told the Guardian she had underpaid her tax and was in talks with HMRC about rectifying the problem.

The Guardian has seen the document filed with the Land Registry to apply to change the name on the deed to the Hove flat. The document was filed by Verrico, under a reference that includes Rayner’s name.

According to its website, Verrico & Associates Ltd is a small firm based in Herne Bay run by Verrico and her three daughters. Experts have said that small, high-street law firms offering general legal services may not have the expertise to deal with the complex law that determines the ownership of properties held in trust. It is not yet known who Rayner relied on for advice on the trust.

Downing Street said on Thursday: “The final legal opinion was received by the deputy prime minister on Wednesday morning, at which point she immediately took steps to refer herself to the independent adviser, and begin the process of engaging with HMRC.”

Magnus could submit his report as soon as Friday, with its outcome likely to determine the deputy prime minister’s political fate.

Rayner received backing on Thursday morning from the chancellor, Rachel Reeves, who said: “I have full confidence in Angela Rayner. She’s a good friend and a colleague. She has accepted the right stamp duty wasn’t paid. That was an error, that was a mistake. She is working hard now to rectify that, in contact with HMRC, to make sure that the correct tax is paid.

“Anyone that saw Angela’s statement yesterday, saw her interview yesterday, I think will have a lot of sympathy with some of the challenging family circumstances around this, around Angela’s disabled son.”

Earlier on Sky News, the education secretary, Bridget Phillipson said: “The deputy prime minister sought advice at the point of purchase of the house, which enacted in line with the advice that she received.

“The deputy prime minister has been clear for some time that she believed she had acted in good faith, that she had paid what was required of her through that house purchase. It then became clear subsequently that that wasn’t the case, that additional stamp duty was owed.

“That was following new, fresh legal advice, but as I say there were limitations on what could be discussed given the existence of a court order that was there to protect her family and to protect her son.”

On Wednesday night, Rayner’s Hove flat was vandalised with graffiti saying “tax evader”, an act her spokesperson called “unjustifiable and beyond the pale”.

• This article was amended on 4 September 2025. Hove is in East Sussex, not West Sussex as an earlier version said.