(REUTERS) - RBS and its retail banking unit Netwest's online systems and debit cards failed on Monday evening as customers were unable to process payments on one of the busiest online shopping days called the "Cyber Monday".

Millions of customers across the United Kingdom were unable to withdraw cash, pay for goods or use telephone and online banking services.

In its response, the bank's twitter account, RBS Help tweeted "we are aware of some technical issues and are working hard to fix them. Sorry and thanks for your patience." The bank's Irish unit, Ulster Bank, also reportedly faced similar glitches.

RBS, which is 82 per cent owned by the UK government, faced a probe by the Financial Conduct Authority in April when technology failure left customers unable to avail the bank's card and online services.

A software upgrade gone wrong also led to similar problems for RBS' customers in June 2012.