The fine relates to its First Class Post Paid Impression and First Class Response Services used by businesses and follows an enforcement order Postcomm issued to the postal operator in December, calling for it to improve its service.
Postcomm applied the fine after it decided that Royal Mail failed to sufficiently improve the services.
Postcomm chairman Graham Corbett said: "Most businesses lose customers and lose money when they provide poor service. I hope that this penalty will drive home to people at all levels in Royal Mail the message that customer service matters."
Royal Mail and others have 28 days to make representations to Postcomm in relation to the proposed penalty.
Royal Mail is required to meet delivery target of 92.5% for both services for post the next day. However, it failed, with 86.3% for the PPI service and 86.8% for the response service.
Royal Mail chief executive Adam Crozier responded to the fine with a statement that the penalty would increase the "financial pressures on the company" and warned that it could further delay pay rises for its workers.
Crozier said: "Postcomm's decision on the fine carries a stark message for everyone in Royal Mail as the CWU plans a 24-hour strike in London. It is unlikely that the regulator will make any allowance for falls in customer service caused by a strike."
He added that some of the London postcode areas affected by the one-day strike are "among the worst performing" in the country and called it "madness" that the union is to inflict "further financial damage" through strike action, which begins today, October 1.
He said: "[Today's] 24-hour strike in London could cost Royal Mail up to £10m. As Royal Mail has said repeatedly to the CWU, a strike will not produce more money to increase the substantial amounts of London weighting payments already on the table, which will put postmen and women in the top third of any London weighting league table.
"There is no more money to come. Today's fine simply means that Royal Mail will have to work even harder to generate the promised rises in pay and London weighting."
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