The City of London is lobbying hard against a windfall tax on the banks, as Labour seeks to balance the books.

Questioned on the departure from the U.K. of billionaire Chris Rokos — who pays £330 million a year in tax to the U.K. — Reynolds maintained that "we have to have rules."

"We want to create the very best conditions for growth, for entrepreneurship," she said. "But we have to have rules, and people who make a living here should pay taxes here."

The Treasury minister would not be drawn on the government-wide budget, which will be announced in October, but said ministers are "not powerless" in dealing with financial insecurity caused by international crises.

"We do face big global economic headwinds and a very difficult geopolitical situation with the ongoing war, particularly in the Middle East, but also in Ukraine," she said. "But we are not powerless to deal with those things, and we will do all that we can in the way that John [Healey] talked about earlier to further take further action to give families and businesses breathing space."