A second 24-hour strike by Royal Mail staff has taken place as part of a row about pay and job security.
Postal workers staged their first 24-hour strike in June
The Communication Workers Union (CWU) said most of its 130,000 members - more than 90% - had joined "one of the best supported strikes in memory".
But Royal Mail said support had been "patchy" and that all 14,220 post office outlets had wolked "as normal".
The CWU is in dispute with Royal Mail over the company's 2.5% pay offer and modernisation plans.
The 24-hour walk out followed a stoppage last month, which had been the first in more than a decade.
About 100 striking postal workers travelled on an open-top London bus at noon to deliver an oversized letter to Royal Mail executives at the company's headquarters demanding a return to proper negotiations.
"This is the only letter being delivered in the UK today and it is a very crucial one in that the future of the company and the postal services rests on it being read," CWU general secretary Billy Hayes told strikers.
Royal Mail denied that the CWU's symbolic act was the only UK mail delivery, arguing the percentage of postal workers coming to work "ranged from 5% to more than 60% around the country" - a better turnout than the previous 24-hour strike two weeks ago.
The government could now get involved in bringing the two sides to the negotiating table, with about 60 MPs adding their names to an early day motion tabled by Labour MP for Morecambe & Lunesdale, Geraldine Smith, calling for an end to the deadlock.
In a statement, Mr Hayes said: "We are consistently trying to negotiate with Royal Mail but to be blunt they have no interest. They refuse to take the dispute seriously."
Union officials are due to meet again next Tuesday, when more strikes could be announced.
The union has argued that workers' pay should rise in line with inflation, which Royal Mail insists it cannot afford amid a "serious competitive" threat from a growing number of more efficiently-run rivals.
The Royal Mail's modernisation plans, for which £1.2bn of public funds have been earmarked, are also a bone of contention, as the CWU says these will lead to the loss of 40,000 jobs.
But the company says that changes are essential in order "to secure a future for the company and its people".
"Yet again the union has refused to grasp or understand the harsh commercial reality of the market in which Royal Mail now operates and the consequences for all of us if we don't modernise - and do it quickly," said Royal Mail chairman Alan Leighton.
Since Royal Mail lost its monopoly status on post deliveries at the start of 2006, 17 operators have entered the UK mail market, creating fierce competition for the former monopoly - particularly in the more profitable business post sector.
UK MAIL MARKET
The UK mail market is shrinking by 2.5% per year
Royal Mail has lost 40% of corporate business
Royal Mail rivals will handle one in five of all letters posted in the UK
Source: Royal Mail
Royal Mail says it has already lost about 40% of its bulk mail business to rival postal operators, including recently an £8m contract with Amazon.
And some private mail firms are actively cashing on the industrial action affecting Royal Mail's service.
One of the group's largest rivals, DX, has offered to help Royal Mail make its urgent deliveries over the period of the strike.
The company told the BBC it expected to gain about £10m of business as a result of the strike action, although it expected the damage to Royal Mail to be minimal.