The prime minister must launch an investigation into Sajid Javid’s time as a banker, John McDonnell has demanded, accusing the chancellor of being implicated in “some of the worst excesses of the casino economy”.

In a letter to Boris Johnson, the shadow chancellor called for a probe into Javid’s 18-year career in finance before he became a politician, taking aim at his time at Deutsche Bank in the run up to the global financial crisis.

Meanwhile, McDonnell said the chancellor – who was appointed in Johnson’s cabinet reshuffle last month – must answer questions about any connection he may have had with the ‘dark blue’ tax avoidance scheme. 

The Labour frontbencher wrote in the letter – which has been seen by the Guardian – that it “will not be lost on those that have suffered the consequences of the last nine years of austerity following the 2008 financial crisis that the newly appointed chancellor profited from the greed that contributed to it.” 

The PM must reconsider Javid’s fitness to serve as chancellor, McDonnell added. 

But a Conservative spokesperson told the paper that Labour “might want to use the time better thinking about their own credentials for governing”, calling the party “the only threat to the UK economy”. 

“Their reckless plans would see debt soaring as they spend one thousand billion pounds, tax raids on hard working families and upheaval of our economic system by replacing key posts such as the governor of the Bank of England with their hard left choices,” they said.  

“Utter disaster. If any party knows about failure and bogus investigations, it’s them.”