The Royal Mail is launching trials of parcel deliveries and office openings on Sundays, it has emerged.
The parcel deliveries will all be made within the M25 area and customers will be able to collect them from around 100 offices at the end of the week.
Royal Mail said the move was aimed at making it easier for online shoppers to collect their purchases if they were not at home on a weekday.
Offices will be open for four hours from noon on a Sunday.
Nick Landon, managing director of Royal Mail Parcels, said: "We are continuing to be more customer-responsive and provide more options for people to receive items they have ordered online."
Changes to collection times at thousands of Royal Mail post boxes adds "insult to injury" after its privatisation, the shadow business secretary said.
Up to 50,000 of Royal Mail's post boxes will be moved to an earlier collection time between 9am and 3pm.
"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," Labour's Chuka Umunna said.
"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."
Royal Mail is to cut its late collection times at post boxes by almost half.
Between 45,000-50,000 of its 115,3001 post boxes will be moved to an earlier collection time between 9am and 3pm.
The company said there will be a late posting box within half a mile and have pledged to add 2,000 more postboxes to its network.
Royal Mail said the changes will "improve the efficiency of its collection arrangements".
Shadow business secretary Chuka Umuna said the changes are "disappointing" for customers and businesses.
A committee of MPs says the taxpayer missed out on an extra £1 billion when Royal Mail was privatised last year.
The Commons business select committee says shares were undervalued, but Business Secretary Vince Cable dismissed the findings as "hindsight".
ITV News Political Correspondent Libby Wiener reports.
There were indications before the sell-off of Royal Mail began of demand "really taking off", according to the chair of the business, innovation and skills select committee.
Labour's Adrian Bailey dismissed claims criticism of the sell-off was all in hindsight and told Good Morning Britain the Government failed to act on clear signs of high demand for shares.
"There were indications before the float that demand for these shares were really taking off. Institutional investors over-subscribed it by 24 times and retail investors...by seven times."
MPs have accused the Government of shortchanging taxpayers out of £1 billion over the Royal Mail sell-off but the Business Secretary said it was the best price they could get at the time.
- The Government sold shares in Royal Mail at 330p
- The all-time high was 618p per share
- Royal Mail shares are currently trading at 473p
Vince Cable said MPs criticising the Government over the privatisation of Royal Mail have "the benefit of hindsight".
The Business Secretary said: "We sold at a price that was regarded as the best that could be achieved in the context in which we sold it."
"The point we have stressed ... is the price of shares is very, very volatile. These things go up and down."
"I think the wisdom of hindsight about price doesn't really help us very much."
The Government pressed ahead with "an unnecessary and botched fire sale of Royal Mail," the shadow trade minister said.
Ian Murray said as a result of this, taxpayers "have been short changed by hundreds of millions of pounds while the Government's 'priority' City investors made a killing at the public's expense."
Mr Murray said: "The cross-party committee's damning report reinforces the significant criticisms which have already been made by the National Audit Office and others.
"David Cameron's Government still has serious questions to answer."
A select committee said they were "disturbed" that the Government may have failed to reap the benefits of privatising the Royal Mail.
The MPs said Royal Mail's assets included three sites in London valued by the Business Department at around £200 million, which the National Audit Office said possessed a "hidden value" worth £330 million to £830 million.
The committee found the Shareholder Executive, Lazard - the Government's financial adviser - and UBS and Goldman Sachs - the Government's global co-ordinators - failed to gauge demand at higher price levels and did not give appropriate consideration to maximising value for money for the taxpayer.
Taxpayers have lost £1 billion over Royal Mail's privatisation because the Government underestimated demand for shares, an influential group of MPs has reported.
Ministers were accused of being afraid to fail over last year's sell-off and of receiving "poor quality" advice.
In a hard-hitting report, the Business Select Committee said taxpayers were missing out on "significant value."
The advice ministers received on the sale was not up to standard, the MPs concluded.