Changes to collection times at thousands of post boxes are "disappointing" for consumers and businesses, the shadow business secretary has said.
Royal Mail said the changes, which will see mail picked up by delivery staff during their rounds, will ensure it is able to maintain its estate of 115,000 post boxes, despite a 33% drop in volumes since 2005.
Between 45,000 and 50,000 of its post boxes will move to the earlier collection time - typically 9am to 3pm - although the company has pledged there will still be a late posting box within half a mile.
But Labour's Chuka Umunna said the announcement added "insult to injury" after Royal Mail was privatised last year.
"In their unnecessary fire sale of Royal Mail, the Tory-led government put vital postal services at risk and, as many feared when the privatisation took place, we are now seeing consumers losing out," said Mr Umunna.
"This adds insult to injury after taxpayers were left short changed by hundreds of millions of pounds as the 'priority' City investors selected by the Tory-led government made a killing.
"Labour is clear that the Universal Service Obligation and the historic link between Royal Mail and the Post Office network must be safeguarded and strengthened to protect postal services for the future."
Royal Mail has also said it will introduce 2,000 new boxes in areas of under-provision, particularly in rural parts of Scotland and Northern Ireland.
Some of the new boxes will also be added in high footfall areas such as railway stations and shopping centres.
Around 12,000 postboxes in rural areas are already emptied through collection on delivery, but the majority of those affected by today's changes will be in urban and suburban areas. An indicator tab on the post box will show if the collection has been made.
Royal Mail said the changes will unlock the opportunity for efficiency savings, while adding that many post boxes no longer cover their costs.
It said: "Rather than decommission uneconomic postboxes, while staying within the regulated density requirement, Royal Mail will ensure their viability by improving the efficiency of its collections arrangements."
The company said it has discussed its plans with Citizen's Advice and Consumer Council Northern Ireland and has also notified Ofcom.
The regulator requires that 98% of all households should be within half a mile of a post box. With the addition of around 2,000 new post boxes, the company said it will improve on its current rate of 98.32%.
Last year, Ofcom's User Needs Review found that 91% of postal users did not choose a postal box based on collection time.
An Ofcom spokeswoman said: "Ofcom recognises the need for Royal Mail to become more efficient so it can sustain a universal postal service that consumers value highly.
"While the changes won't affect the majority of postal users, Ofcom expects Royal Mail to communicate clearly with any affected consumers and ensure that their reasonable needs continue to be met."