Angela Rayner should not have escaped a fine for underpaying stamp duty on her seaside home, according to a Government expert in the department that investigated her.

The former Deputy Prime Minister announced last week that she had been 'exonerated' over the underpayment of stamp duty on the £800,000 home in Hove, though she has had to pay an extra £40,000 in duty on the property.

Ms Rayner's announcement at 6am on Thursday distracted attention from that day's Cabinet resignation by Health Secretary Wes Streeting – a potential rival for leader of the party – in protest over the leadership of Sir Keir Starmer.

HMRC launched its inquiry last September after it was revealed that she had paid £30,000 stamp duty for the apartment, rather than the £70,000 required on a second home.

Ms Rayner says that the investigators accepted that she had taken 'reasonable care' when she paid the lower sum.

But the whistleblower, who until recently worked in HMRC's Fraud Investigation Service, has expressed 'concern about the Angela Rayner situation' and told The Mail on Sunday: 'A penalty can be imposed for a careless error, a deliberate error or a deliberate error that is then concealed. The maximum penalty is higher for deliberate errors than for careless errors and higher again if the deliberate error is concealed.

'Rayner has specifically said that she has been found not to have deliberately avoided the tax. I do not dispute this. On the known facts the error was not deliberate – it was careless.'

Ms Rayner was forced to resign from the Cabinet last year after Sir Laurie Magnus, the Prime Minister's ethics adviser, concluded she was 'careless' over the tax bill.

The whistleblower said that the usual HMRC conclusion in such a case of apparent 'carelessness' would be to levy a fine, but suspend it on condition of 'good behaviour' in future.

A spokesman for Ms Rayner said that she had not been hit with any penalty, suspended or otherwise.

But the whistleblower added: 'It still raises the question of why HMRC agreed that she took reasonable care.'

The Conservatives have demanded that Ms Rayner should share publicly the evidence provided to HMRC, after Graham Aaronson KC, Ms Rayner's lawyer, said that 'new facts' had come to light that allowed her to evade a penalty.

Ms Rayner has now paid the additional £40,000 she accepted was owed on the purchase of her £800,000 home in Hove in 2025.

The former housing secretary and deputy PM said that the HMRC probe had 'clipped her wings' as speculation mounted about a challenge to Sir Keir – and whether she would stand in her own right or on a 'dream ticket' with Andy Burnham.

The MP for Ashton-under-Lyne did not rule out running, but said she would not 'trigger' a contest.

She said: 'I'll play my part in doing everything we possibly can to deliver the change, because it's not a personal ambition, I know the difference it makes.'

Dan Neidle, a tax expert, said: 'If Ms Rayner wants people to accept that she acted properly, and HMRC's decision was correct, then she should release the evidence that led HMRC to conclude she was not careless.'

Ms Rayner has denied that the timing of the HMRC announcement means that she benefitted from a 'fast-track sweetheart deal', and had use of a VIP hotline known as 'Public Department 1', which is reserved for members of the Royal Family, MPs and the ultra-rich.

A spokesman for Ms Rayner said: 'Angela had no need to make use of a VIP helpline in this matter. She instructed professional advisers.'