Rachel Reeves is caught in a grim trap of Labour's making. She cannot trim public spending because her own backbenchers will rebel, just as they did over cuts to disability benefits. Borrowing more is also off the table. With the national debt nearing £3 trillion, bond markets won’t tolerate it.

Reeves won’t break her “iron-clad” fiscal rules either. This leaves just one way to plug the £50billion black hole she now faces. But it’s politically explosive and the backlash could sweep her away.

There are many taxes she could theoretically hike in November’s Budget, but most will do more harm than good.

Reeves daren’t pile even more taxes on businesses. Last year’s £25billion hike in employers’ national insurance has already destroyed 276,000 jobs, with another 100,000 gone by year-end.

The added burden has pushed thousands of shops, pubs, hotels and restaurants closer to collapse, ultimately hitting tax receipts rather than boosting them.

Other tax avenues are blocked too.

The left is clamouring for a wealth tax, which it reckons will raise £10billion. But like the raid on foreign non-doms, it's likely to backfire by driving away the better off. It would also be insanely complex to set up which means Reeves wouldn't see a penny for years.

Levies on tobacco, alcohol and gambling may rise, with estimates suggesting a gambling tax raid could raise £3billion. But even that won’t close the gap.

A windfall tax on bank profits would antagonise the City and contradict Labour’s efforts to woo finance for growth.

Slashing pension tax relief could raise a fair sum and might be on the table, but it’s no silver bullet.

That leaves three big taxes with the heft to fill the hole: income tax, national insurance and VAT.

But each brings its own problems. Not least that hiking any of them would break Labour's key manifesto pledge not to increase taxes on "working people".

So which could she choose? Each 1% rise in VAT would raise around £8billion a year but push inflation higher. As a flat-rate tax, VAT hits the poorest hardest. Backbenchers won’t like that.

Reversing the Tory's pre-election national insurance cuts from 12% to 8% could raise £20billion a year. Yet NI is capped at higher levels, so lower-paid workers would pay proportionally more. Not a good look for Labour.

Which only leaves income tax.

Reeves is widely expected to extend the freeze on income tax thresholds for another two years, until 2030. But that won’t help her today.

Hiking income tax rates from next April would.

The National Institute of Economic and Social Research says she would need to raise basic and higher rates by a punishing 5% to plug her black hole. I doubt she’ll go that far, but a 1% or 2% rise is becoming likely.

The last Chancellor to increase the basic rate income tax was Labour’s Dennis Healey, way back in 1975. Fifty years later, the Institute for Fiscal Studies says Reeves should bite the bullet as "drastic times call for drastic action".

But it would be a nightmare. Income tax is the most politically toxic tax of all, by far the biggest deduction on the payslips of 30million workers. And Reeves would catch all of the flak.

In her defence, Reeves is the only Labour frontbencher who seems to grasp the scale of the fiscal meltdown.

She’s made mistakes, but her own party’s refusal to face reality has pushed her into an impossible position.

Quitting now would send a wake-up call to Labour, forcing it to confront the harsh economic reality Britain faces.

Reeves will no doubt plough doggedly on. It will be painful to watch, especially for taxpayers.