Britain's Royal Mail has "systemically failed" to meet its mail delivery requirement, lawmakers said Friday, calling upon regulator Ofcom to probe the postal operator.

"We believe that Royal Mail has systemically failed to deliver against parts of its universal service obligation," read a report from the cross-party Business, Energy and Industrial Strategy Committee.

The obligation requires the Royal Mail to deliver letters six days a week in a one-price-goes-anywhere postal service.

However, the committee said it found "widespread evidence" that Royal Mail prioritised parcels over letters without acknowledging the policy change.

"We recognise the challenges of both the pandemic and ongoing industrial action, but the evidence we have suggests this systemic failing has been taking place before, between and during these events," it added.

The cross-party parliamentary committee has asked Ofcom to launch an enforcement investigation that will report back by the end of this year.

"Ofcom must start enforcement proceedings to ensure everyone gets a consistent service wherever they are," said committee member and opposition Labour MP Darren Jones.

"Otherwise, what's the point in having a universal service obligation at all?"

Set up more than 500 years ago, Royal Mail experienced turbulent times during the past decade, particularly following its privatisation in 2013.

The former state-run operator was boosted during the pandemic lockdowns by a high volume of parcels but this has since fallen sharply.

And it was blighted by industrial action last year as workers protested at wages that have failed to keep pace with soaring inflation.

Royal Mail's parent group International Distributions Services said in January that it suffered an operating loss of £300 million ($363 million) in the nine months to December, after taking a vast hit of £200 million from the strikes.

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