John McDonnell’s borrowing plans would cost the taxpayer £270 billion in debt interest alone over the course of just one five-year parliament, new research suggests.

The shadow chancellor’s extra spending proposals would mean a Labour government borrowing £330 billion, adding to the £1.8 trillion of current national debt.

Debt interest payments would add up to more than £270 billion, according to the analysis undertaken by the TaxPayers’ Alliance. It calculated the figures based on the interest rate on government debt forecast over the period.

It comes after Mr McDonnell said any increase to the national debt would “pay for itself” and refused to answer questions about figures for his borrowing plans in a radio interview last week because MPs had “iPads and advisers” for that.

John O’Connell, chief executive of the TaxPayers’ Alliance said: “The shadow chancellor’s claim that borrowing plans would ‘pay for itself’ is entirely wrong. Spending pledges in his manifesto, even when taking into account revenue raised from proposed tax hikes, would cost Britain more than £270 billion in debt interest payments over the parliament.

“Britain is up to its neck in debt, yet politicians of all parties continue to make extraordinary spending pledges that puts huge pressure on taxpayers.”

Mr McDonnell apologised if he came across as arrogant in the interview on BBC Radio 4’s Today.