Labour has appointed an expert panel to help it “modernise” His Majesty’s Revenue and Customs (HMRC) if it wins the next election, with the shadow chancellor pledging to stop people being “left hanging on the phone”.

Speaking to Sky News, Rachel Reeves said the panel would advise her on how to improve tax compliance and “bring in this additional money” to the government’s coffers, as well as updating the technology behind the service.

And she said the group would make recommendations on “how we can modernise the HMRC and make it a better experience for people phoning up… who are often left hanging on the phone because there’s no one answering the calls”.

The announcement comes just weeks after a U-turn from HMRC over its plans to shut its helplines for half of the year, following an outcry from the public and politicians.

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It also comes as Labour revealed how it would fill the gaps in its spending plans after the Conservatives stole two of their revenue-raising policies – the scrapping of the non-dom tax status and an extension of the windfall tax on oil and gas firms – to pay for a cut in national insurance.

Ms Reeves said the party would raise £5bn by the end of the next parliament to fund breakfast clubs for primary school children and additional appointments in the NHS by clamping down on tax avoidance and closing “loopholes” in the government’s own non-dom pledges.

Put to her that £5bn was a small fraction of public spending overall, the shadow chancellor insisted it was not “a drop in the ocean”, telling Sky News’ Tamara Cohen: “I think that will make a big difference to the lives of hundreds of thousands of people across our country and that is the difference that a Labour government will bring.”

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She added: “[The] Conservative government is still not willing to properly clampdown on tax avoidance or to ensure that our non-doms are paying their fair share.

“It’s [a] Labour government that is willing to make those tough decisions and put that money where it’s needed into our frontline public services.”

Ms Reeves warned there would be “constraints in what an incoming Labour government would do”, blaming the Tories for playing “fast and loose with the public finances”.

But she insisted everything in the party’s manifesto would be “fully costed and fully funded”, adding: “People can have the certainty and the security of an economy well-run, after the chaos they’ve seen these last few years.”

Asked if any tax cuts would be on the cards with her in the Treasury, Ms Reeves said: “I want the tax burden on ordinary working people to be lower, but I won’t make any promises that I can’t say how I will keep.”

But she did reiterate Labour’s commitment to the triple lock on pensions, and hinted an emergency budget could come quickly if her party gets the keys to Number 10 and 11.

Ms Reeves confirmed her first budget would include measures to close non-dom loopholes and the windfall tax extension, and Sky News understands it will also include its policy of imposing VAT on private school fees.

“Of course… we will have to bring forward a budget,” she said. “There will also be a need for a spending review.

“But we’ll take one thing at a time. We need to win the election and then we will be able to put into practice the policies.”

Ms Reeves was also asked about the ongoing row around Labour’s deputy leader Angela Rayner and her living arrangements before she became an MP.

Ms Rayner is facing accusations she avoided capital gains tax after selling her property in Stockport in 2015, and falsely registering to vote there while living at her then husband’s house nearby – claims she has denied.

Asked if she had seen the tax advice given to Ms Rayner, the shadow chancellor said: “ No, because I haven’t seen the tax returns or the tax or legal advice of any of my colleagues, and I wouldn’t treat Angela Rayner different to other colleagues.

“But she has taken that advice as she is confident, and I have every faith and trust in my friend and colleague Angela Rayner.”