“When I was mayor of London, I cut my share of council tax by 20 per cent”

That’s what Boris Johnson told reporters this week. He’d been challenged to defend his status as a “low-tax Tory” after announcing a manifesto-busting increase to National Insurance to help fund social care.

But looking back at Mr Johnson’s time in City Hall, it’s hard to see where that 20 per cent figure comes from.

As mayor, he was in charge of setting one part of Londoners’ council tax bill, known as the mayoral precept.

In his first budget for the capital in 2009, Mr Johnson decided to maintain the precept at the level set by his predecessor, Ken Livingston, charging the average “Band D” property £309.82 a year. And he kept it at that rate for the next two years.

In 2012-13, he started to bring it down by a few pounds each year, until a final drop of £19 in his last budget in 2016.

By the time Mr Johnson left office, the mayoral precept for Band D properties was £276.00 – some £33.82 lower than when he started.

But that’s a cut of 10.9 per cent over his tenure, not 20 per cent as he claimed this week.

It could be that the prime minister has another way of measuring the precept in mind. But during his time as mayor, his own team used Band D properties to track changes to council tax, just as we have in this article.

A budget document prepared by his office in 2015-16 described the plans to cut Band D rates in his final year as “equat[ing] to a 10 per cent reduction in the precept over the Mayor’s second term”. And as we’ve seen, rates were all-but flat in his first term, so it seems the 10 per cent figure (or 10.9 as we’ve calculated) applies across his whole tenure.

Downing Street was contacted for comment.

FactCheck verdict

Boris Johnson said that as mayor of London, he cut his share of council tax by 20 per cent. But according to the standard measure of the mayoral precept – the same one used by his team when he was in City Hall – the true figure is 10.9 per cent.

Update: 10 September

Since we published this article, readers have suggested that perhaps adjusting the mayoral precept for inflation might explain Mr Johnson’s claim. There are various ways to do this, and all of them would make the scale of the cut larger than 10.9 per cent.

However, people with knowledge of how the mayor of London’s accounts are prepared have told us that while accounting for inflation is common when we’re talking about national public spending, the tendency in local government is to use “cash” figures, i.e. not inflation-adjusted. This approach is not perfect, though it seems to be the one that Mr Johnson’s team used when he was mayor.

We’ve yet to hear from Downing Street what the prime minister had in mind when he spoke this week.