The Post Office is promising overnight delivery for letters with a fivepence stamp on, while fourpence buys you a slower service.
It hopes to raise an extra £25 million from providing the new service, as well as levelling out the workload for postmen.
It stands to add to the complication of what is already a complicated operation
Marcus Young, National Provincial Bank
At one central London post office, there were long queues as counter clerks explained the new system - to the dismay of some customers.
"I just don't think it's worth the increase," said one man. "Not when you see all these queues here, a number of positions not manned and people standing around. I just don't think it's worth it."
The biggest impact has been on businesses, which account for about 75% of all mail.
John Pegnall, the secretary of the National Chamber of Trades, said most were angry at what he called a "confidence trick" played on them by the Postmaster General, John Stonehouse.
"He's rather hidden the fantastic increase in charges - 33.3% in that particular rate... in the pretence that there's a fantastically good new service being introduced, when there's nothing of the kind," he said.
At the National Provincial Bank, which sends out millions of letters each year and spends over £500,000 on postal charges, there was concern over the extra costs in addition to the expense of a first-class service.
"A decision has got to be taken for every letter for which post it should go by, the envelope has to be marked, the postal clerk has to see this mark and stamp it accordingly, and so on," said the bank's deputy Chief General Manager, Marcus Young.
"So it stands to add to the complication of what is already a complicated operation."
Several businesses, including the BBC, Esso, British Petroleum, and Gillette, have instructed staff not to send letters by the 5d rate unless absolutely necessary.
But after the first day, the Post Office said it was encouraged by the take-up for the new rate of postage.
Advert on the side of a post office van
Post office customers are being asked to choose between two prices of stamp
In Context
Just over a quarter of all letters posted on the first day were posted at the first-class 5d rate - well short of the Post Office target of 40-45%.
The initial take-up of the first-class rate has stayed roughly the same: in 2003 about 30% of mail handled by the Post Office was being delivered by first-class post.
However, since 1968 the number of letters the Post Office delivers has almost tripled, from 30 million to 82 million each day.
Meanwhile, the cost of sending a letter first class has risen in real terms by nearly a quarter, although the price of a second-class stamp has stayed almost the same.
But whereas in 1968 the Post Office aimed to get 95% of first-class letters delivered by the next day, in 2002-2003 the target was down to 92.5% - and the Post Office failed to reach it by 0.7%.
The Postmaster-General, John Stonehouse, later became notorious for faking his own death. He resigned as an MP after he was convicted for fraud in 1976 and sentenced to seven years in jail.
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