The head of Britain's statistical watchdog said Prime Minister Rishi Sunak should be clearer about the source of his claims on the opposition Labour Party's tax plans, as he may have given people the "wrong picture" on how independent they were.

Labour accused Sunak of lying during an election debate with its leader Keir Starmer on Tuesday. Sunak said "independent Treasury officials have costed Labour policies" and these plans would lead to a 2,000 pound ($2,560) tax rise for working households.

Sunak's Conservatives are far behind Labour in opinion polls ahead of the July 4 election, but a snap poll after the debate showed viewers thought Sunak had performed slightly better than Starmer.

Sunak's 2,000 pound figure - which represents a cumulative cost over four years - came from a Conservative Party document based largely on cost estimates made by Treasury officials and Labour's own costings. However, many key assumptions used by the Treasury about how exactly Labour policies would be implemented were set by Conservative Party advisors who work in government.

Robert Chote, chair of the UK Statistics Authority, said Treasury costing of opposition policies was a practice that had existed in British government since the 1950s, which he personally thought should be scrapped.

"This was a bad precedent when it started and we'd be much better off without it," he told the BBC.

Asked if Sunak had been clear enough about the origin of the figure, Chote said: "When you've got people getting the wrong picture about the confidence, the independent verification that goes behind this, that's not a great place to be."

Speaking to Times Radio, work and pensions minister Mel Stride said the Conservatives stood by the 2,000 pound figure, and that Labour had taken a similar approach to Conservative policies when it was last in power in 2010. ($1 = 0.7818 pounds) (Reporting by David Milliken; Editing by Toby Chopra)