Perhaps the most important part of my new TUC report on the 50p tax rate is my new estimate of the tax that this rate might raise.

The figure I have estimated is that this tax will raise at least £6 billion in revenue in the tax year 2011/12. The calculation is, however, not really mine. This is H M Revenue & Customs' estimate. I have based my figure on HMRC's estimates of tax revenue to be raised by tax band for 2011/12, here. I extrapolated this data as follows:

Note my resulting total is £6.8 billion. I've downgraded it to £6 billion to be cautious. But this is still HMRC's data. It is their estimate of the average income of those in each of the tax brackets. It's their estimate of the number of taxpayers. It's their estimate of total income. So candidly, this is really their estimate of the tax to be raised. And it's big - much bigger than anything else anyone has said.

We know HMRC are preparing an estimate of how much this tax rate will raise. But in effect they've already done it: I reproduce it above.

If this isn't the number they publish then the question to be answered is why it is different? But for now, whatever others say, this is HMRC's best estimate of the tax the 50p tax rate will raise. My job's simply been to disclose what they have been thinking all along.