Chancellor Rachel Reeves has been urged to "reject the path of least resistance" and consider increasing taxes in the budget.
The Institute for Government (IfG), a leading think tank, said Labour's "rash" and "unrealistic" approach to tax has left the chancellor reaching for "piecemeal changes".
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The government has repeatedly said it will not increase VAT rates, income tax or national insurance at the budget in November.
But a report by the think tank calls on Ms Reeves to commit to serious tax reform, instead of reaching for an "eclectic grab bag of tax raisers", which could further complicate the system.
It comes after the Resolution Foundation proposed a 2p cut in national insurance , matched by a 2p rise in income tax, to creat