The UK’s statistics watchdog has warned the Conservatives over Rishi Sunak’s claim that Labour would raise taxes by £2,000, saying it failed to make clear how the figures were calculated.

In a rap over the knuckles for the prime minster, who made the claim during a leadership debate on ITV on Tuesday evening, the Office for Statistics Regulation (OSR) said the case demonstrated how all party campaigners needed to offer the public a transparent view of tax and spending policies.

The claim caused an increasingly bitter war of words between Labour and the Conservatives, after Sunak repeatedly said during ITV’s head-to-head debate with Keir Starmer that “independent Treasury officials” had costed Labour’s policies “and they amount to a £2,000 tax rise for everyone”.

The regulator said it was concerned that it was unclear the £2,000 figure referred to a four-year period. It was also concerned that the figure was described as being independently calculated by Treasury officials when some of the data was “derived from other sources or produced by other organisations” that “should not be presented as having been produced by the civil service”.

In a statement on its website, the OSR said: “Without reading the full Conservative party costing document, someone hearing the claim would have no way of knowing that this is an estimate summed together over four years. We warned against this practice a few days ago, after its use in presenting prospective future increases in defence spending.”

It added that refraining from using figures that can be misinterpreted “will avoid the need for subsequent clarifications and corrections and will help build and maintain trust in their claims and statistics overall”.

The chair of the UK Statistics Authority, Sir Robert Chote, who oversees the OSR, said ahead of the statement that the row highlighted the need for politicians to use statistics transparently in the run-up to the election.

Speaking on BBC Radio 4’s Today programme on Thursday, he said: “If you’re going to make high-profile numerical claims about things, then it’s a good idea to define clearly what it is that you’re talking about and you’re trying to measure.

“You should say how you’ve calculated the numbers, you should identify the source that the underlying figures, the underlying data, comes from. You should explain to people what confidence they can have [in] that and what uncertainties might lie around it.

“And in particular, in cases where you distil a whole lot of complicated analysis into a single number or a soundbite, it’s always a good idea to ask yourself ‘Would the average person in the street hearing this number have a realistic chance of understanding what it means, what its significance is, without having to hand them eight pages of explanatory material?’”

Chote added: “In this situation, the prime minister basically said or implied that this had been signed off in total by the Treasury – the Treasury permanent secretary has himself said that that wasn’t the case. And clearly having that in dispute is not great for overall trust in the dialogue and the debate as a whole.”

The Labour party campaign coordinator, Pat McFadden, tweeted: “Gets worse for the Tories. Their ludicrous claim pounded into dust.”

The Conservative party has been approached for comment.