Rachel Reeves has said she is determined to “defy” forecasts that suggest she will face a multibillion-pound black hole in next month’s budget.

Writing in The Guardian, the chancellor argued the “foundations of Britain’s economy remain strong” – and rejected claims the country is in a permanent state of decline.

Reports have suggested the Office for Budget Responsibility is expected to downgrade its productivity growth forecast by about 0.3 percentage points.

That means the Treasury will take in less tax than expected over the coming years – and this could leave a gap of up to £40bn in the country’s finances.

Ms Reeves wrote she would not “pre-empt” these forecasts, and her job “is not to relitigate the past or let past mistakes determine our future”.

“I am determined that we don’t simply accept the forecasts, but we defy them, as we already have this year. To do so means taking necessary choices today, including at the budget next month,” the chancellor added.

She also pointed to five interest rate cuts, three trade deals with major economies and wages outpacing inflation as evidence Labour has made progress since the election.

Speculation is growing that Ms Reeves may break a key manifesto pledge by raising income tax or national insurance during the budget on 26 November.

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Although her article didn’t address this, she admitted “our country and our economy continue to face challenges”.

Her opinion piece said: “The decisions I will take at the budget don’t come for free, and they are not easy – but they are the right, fair and necessary choices.”

Yesterday, Sky’s deputy political editor Sam Coates reported that Ms Reeves is unlikely to raise the basic rates of income tax or national insurance, to avoid breaking a promise to protect “working people” in the budget.

Sky News has also obtained an internal definition of “working people” used by the Treasury, which relates to Britons who earn less than £45,000 a year.

This, in theory, means those on higher salaries could be the ones to face a squeeze in the budget – with the Treasury stating that it does not comment on tax measures.

Read more: The taxes Reeves could raise

In other developments, some top economists have warned Ms Reeves that increasing income tax or reducing public spending is her only option for balancing the books.

Experts from the Institute for Fiscal Studies have cautioned the chancellor against opting to hike alternative taxes instead, telling The Independent this would “cause unnecessary amounts of economic damage”.

Although such an approach would help the chancellor avoid breaking Labour’s manifesto pledge, it is feared a series of smaller changes would make the tax system “ever more complicated and less efficient”.