Most people who have basic current accounts with RBS and Lloyds are currently unable to use cashpoints run by other banks.
However MPs on the Treasury Select Committee have warned that restricting access to cash machines could lead to an overall reduction in the number of ATMs outside bank branches because fewer people are able to use them.
A report by the committee also warned that free banking could be put at risk if banks start charging rivals' customers to use their machines.
The committee has urged RBS and Lloyds to remove the restrictions.
Barclays (LSE: BARC.L - news) , HSBC (LSE: HSBA.L - news) and Santander (Madrid: SAN.MC - news) have all made public commitments to continue to provide basic bank account holders with unrestricted access to cash machines in response to the committee's inquiry into the matter.
A spokeswoman for RBS Group said: "RBS will look closely at the committee's report. We remain committed to offering a free basic account for people who may otherwise struggle to access banking services."