Last August RBS revealed that it had followed Lloyds' lead and was preventing around one million basic account holders from using cash machines belonging to other banks.

The part state-owned institutions argued that letting these customers use rival ATMs is unsustainable because they cannot recover the costs incurred each time a transaction takes place.

Having already expressed concerns about the policy's potential for financial exclusion, the Treasury Select Committee has now published a full report on the issue.

The committee says that the cost savings - around £10 per account for RBS and £12 for Lloyds - appear to be relatively small and have potentially disproportionate effects.

"As the main purpose behind the development of the basic bank account initiative was to improve financial inclusion, it is particularly regrettable that this 'beggar my neighbour' approach may lead to the closure or, in the case of cash machines not operated by banks, conversion to charging of existing free to use cash machines," says the report.

Welcoming the commitment of Barclays, HSBC and Santander to unrestricted ATM access, the MPs call on Lloyds and RBS to follow suit.

Andrew Tyrie, the comittee's chairman, says: "Restricting access to cash machines could compromise the network. In certain areas, more than a third of ATMs could be placed at risk if other providers of basic bank accounts were to take similar action or to remove themselves from the Link system...In this instance, the financial benefits to Lloyds and RBS appear relatively small but those affected would be amongst the most vulnerable people in society."