The tardiness is expected to generate a minimum of £110m for HMRC, as late filers incur an immediate £100 fine with the potential for additional penalties.

Late filing includes penalties not only for tardiness but also for delayed payments. A 7.75% annual interest on the tax due, pro-rated until payment, and fines of 5% after 30 days, six months, and 12 months further contribute to the financial repercussions.

Despite the surge in late filings, a record number of 11.5 million people submitted their self-assessment returns on time, attributed in part to the government’s multi-year freeze on tax thresholds. This fiscal policy has led to more individuals being drawn into the tax system, as “fiscal drag” occurs when rising wages and asset values surpass the stagnant thresholds.

HMRC’s director-general for customer services, Myrtle Lloyd, acknowledged the millions who met the deadline and urged those yet to file to do so promptly.