The burden of tax falling on workers and employers has increased as a hotly-debated rise in National Insurance payments takes effect.
Employees, businesses and the self-employed will pay an extra 1.25p in the pound. The government says it will spend it on health and social care.
Opposition MPs said it exacerbated cost of living pressures, but mitigation will be introduced in July.
Experts are urging people to check their status as a new tax year starts.
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