Post Offices will open for longer, allow customers to access their current account, and offer information for jobseekers, under government plans.
There will be no new closure programme for the network of 11,500 branches, said Postal Affairs Minister Ed Davey.
Customers will be able to use counters and computer screens to access their current accounts held with other banks.
But the option for the Post Office to offer banking services in its own right has been rejected.
The plans are part of efforts to modernise the Post Office, which has experienced a steep drop in customer numbers and has come to increasingly rely on government subsidy.
Last month, the government announced that the Post Office was to receive £1.3bn of extra funding over the next four years to reform the network.
"The vision is that the Post Office will become a refreshed, central point of our community life again where it will have more services," Mr Davey told the BBC.
At present, customers can buy savings and insurance products at post offices, but these products are operated by Bank of Ireland.
In addition, about 60% of current accounts - mainly with the state-backed banks - can be accessed from the Post Office.
The government has now announced that this will be extended to people who hold accounts with Royal Bank of Scotland (RBS), which also includes the NatWest brand.
The government said this would extend access to 80% of UK current accounts. Customers of HSBC and Santander customers are currently unable to access current accounts through the Post Office.
However, the proposal for a Post Office Bank - a separate state-owned institution that could offer a range of services - has been described as "time-consuming and extremely expensive" and has been rejected.
A spokesman for the National Pensioners Convention, which wanted a Post Office Bank to be created, said: "This is an extremely short-sighted decision which will put the future of the Post Office network in jeopardy.
"It will deprive millions of old people a secure banking system they would have confidence in."
- The organisation becoming a mutual body - owned by its customers and workers - at some point in the future
- Customers getting some services online or at self-service computer points in branches, instead of queuing up
- Post Office services being offered at the shop till, rather than a separate counter, leading to longer opening hours
- Small, local shops providing some Post Office work
- Documents, such as information for jobseekers, could be provided at Post Offices which would become a type of one-stop shop for community services.
The National Federation of SubPostmasters welcomed the trials for government services at post offices, which could bring in more customers and provide a "vital new revenue stream".
"However, we need to see these schemes develop beyond trial status," said general secretary George Thompson.
"Ministers must therefore ensure that funding is earmarked to allow the nationwide roll-out of these services across the country on a permanent basis."
The Post Office proposals are separate from the plan for Royal Mail - a business that delivers letters and parcels - which is being lined up for privatisation.
Billy Hayes, general secretary of the Communication Workers Union, warned that plans to privatise the Royal Mail would "kill off" post offices.
He said the sale of up to 90% of Royal Mail, coupled with rumours that the payment of benefits is to be handed to a private firm, would deny post offices the "oxygen" they needed to survive.
"If you take away the oxygen of the Royal Mail from Post Office Counters, if you take away the welfare contract, you are going to kill the post office network," he told the BBC.
The typical Post Office branch offers 170 products and services ranging from foreign currency to fishing rod licences, the government's review of the network said.
However, growing use of the internet and direct debit, has reduced visits to branches from 28 million a week in 2000 to about 20 million a week now.
Government funding during the reforms will increase from £180m in 2011-12 to £330m in 2014-15.
A spokesman from the Department for Business said that postmasters' security would not be compromised if they moved from behind a counter to a more open till.
He added that many of these staff already ran retail businesses, so there would not be major job losses if they currently had no retail offer.
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