MPs have attacked a "rush" to offload Crown Post Offices to WH Smith, and urged Barclays to reverse its decision to end over-the-counter cash withdrawals from Post Offices from next year.
The Business, Energy and Industrial Strategy Committee said it planned to "haul" in the bank's bosses to face questions over the impact of the move.
Its report called on Barclays to reverse its decision to stop allowing its customers to make cash withdrawals from Post Office branches from January 2020.
The committee also made a series of recommendations to tackle fears that the "fragile" Post Office network could "collapse" or that future services could be hollowed out.
The MPs called on the government and the Post Office to reconsider closing Crown offices - larger branches usually based on high streets - claiming that WH Smith was poorly rated by consumers, with a record of paying suppliers late.
The government was urged to make a long-term commitment to support the Post Office network, including extending subsidy payments beyond 2021.
Rachel Reeves, who chairs the committee, said: "Post Offices are a crucial public service and perform a vital social role in our struggling high streets, helping to fill the gaps left by retreating banks.
"But our Post Office system is under threat. Sub-postmasters are working long hours and struggling to make a living, and the retailers running Post Offices are finding it hard to make them viable.
"If we want to avoid a bleak future of Post Office closures, the government needs to step forward with a long-term funding commitment beyond 2021 to support the Post Office network.
"The decision by Barclays to stop its customers accessing their own money from Post Offices is a petty penny-pinching move which has triggered a deserved backlash from small businesses and the public alike.
"Barclays should live up to their social responsibilities, execute a swift U-turn and dump this policy."
The MPs said that between 2000 and 2010, some 6,500 Post Office branches were closed, and there are concerns that recent stability could be threatened, not least because of the financial pressures on sub-postmasters.
A survey of sub-postmasters earlier this year suggested that up to one in five Post Offices could close because sub-postmasters were struggling to make a living, the report said.
A Post Office spokesman said: "We share the committee's disappointment at Barclays' decision not to offer the full range of services available through the Post Office in stopping their customers accessing vital cash withdrawal services at our branches from January 8 2020.
"The service is a real lifeline for millions of customers of UK banks.
"We are, though, delighted that 27 banks have recognised that our network of over 11,500 branches, 6,000 of which are in rural areas, offers by far the best solution for easy access to vital cash withdrawals wherever people live in the UK.
"We recognise the hard work our postmasters put in day in day out in serving customers across the country. That's why we have launched a review of postmaster remuneration.
"We are already passing on the increases in remuneration for processing cash deposits resulting from our new agreement with 28 UK banks."
Mike Cherry, chairman of the Federation of Small Businesses, said: "At a time when high streets are struggling, and banks are shutting branches, the role of the Post Office in local communities has never been more crucial, but this important cross-party report makes clear, in no uncertain terms, that our Post Offices are not receiving the support they need."
Gillian Guy, chief executive of Citizens Advice, said: "This move by Barclays shows the fragility of voluntary agreements like the banking framework in making sure consumers have access to banking in their community.
"The Government needs to step up and develop a comprehensive plan so face-to-face banking services continue to be available for those who need them."
The committee announced it will be holding a follow-up session on November 6, questioning Matt Hammerstein, Barclays UK chief executive, and representatives from the Post Office and Which?
A WH Smith spokesman said: "WH Smith is proud to operate over 200 Post Offices within its High Street stores, ensuring vital post office services continue to be delivered on high streets across the UK.
"We are a highly profitable and growing business with over 225 years of heritage and we are committed to providing these important services to the communities we serve."
