The claims relate to an ad that appeared in a national newspaper on 2 November stating: 'You won't find a better rate than the 6.2% you'll get on a monthly saver account at Bank of Scotland (Ireland)'.

However, after a tit-for-tat battle at the end of October, AIB trumped Bank of Scotland by increasing the rate on its rival regular saver account to 6.34%.

It is understood that there have been several complaints about Bank of Scotland's ad to the Financial Regulator, although a spokeswoman said the watchdog could not comment on matters concerning individual financial institutions.

A spokesman for Bank of Scotland said it had received no complaints about the ad, either from the public or the Financial Regulator. He added that there were no plans to drop the ad or change the claims made in it.

As part of its consumer protection mandate, the Financial Regulator continuously scrutinises all advertising by banks and insurance companies. It raised concerns about 105 ads last year and investigated another ten complaints from members of the public.

Three institutions . . . AIB, Bank of Scotland and Anglo Irish Bank . . . are paying rates of 6% and over to win business from regular savers.