MPs have called on the government to take action over Nationwide's planned closure of all its inner southeast London branches.
Nationwide is closing branches up and down the country as it moves to cut costs after buying up troubled Dunfermline, Chester and Derbyshire building societies since the credit crunch in 2008.
The closure of seven London branches will mean 670,000 people, the entire population of Southwark and Lewisham and most of Greenwich, will be unable to get to a branch, Nick Raynsford, Labour MP for Greenwich and Woolwich says.
"This is more than the entire population of a large city such as Sheffield or Manchester," said Raynsford in a Commons debate on 18 March.
"What is particularly galling about this sad process is that Nationwide had its origins as the Co-operative building society, and continues to proclaim its commitments to mutuality."
Mark Hoban, financial secretary to the Treasury, said the government cannot intervene in the opening and closing of banks' branches.
However, he added the government is committed to improving access to financial services to all consumers, and said legislative reform of provident societies and credit unions will help these providers fill the gap left by banks and building societies.
"The government is working with banks, building societies, e-money service providers, bill payment organisations, retailers and post offices to pursue new ways to improve the opportunities for low-income households to make the most of their money," he said.
Two global vehicles
'Further plug advice gap'
Must appoint separate CEOs and boards
Advisers do come out well
Will report to Mark Till