Spanish-owned Santander has been receiving complaints at a rate of one a minute about its UK banking services during the past six months, new figures have confirmed.

The bank, which includes the re-branded high street branches and savings business of Bradford & Bingley, has emerged as the bank with the highest proportion of customer complaints.

Santander received 245,000 complaints during the period, making it the most complained about UK bank, followed by Barclays with 195,956 complaints.

The Co-operative bank, part of the business group headed by Bradford-born chief executive Peter Marks, had the lowest ratio of complaints with 2.1 for every 1,000 accounts.

The figures have emerged following a ruling by the Financial Services Authority that UK banks had to publish the complaints data on their websites by August 31. Although the FSA has not complied a ‘name and shame’ list, a spokesman said the watchdog was planning to do so.

Santander said the number of complaints it received was seven per cent lower than in the same period of 2009, and was equivalent to around 1,300 complaints per day.

Around seven out of every ten complaints were dealt with within 48 hours and only two per cent were outstanding for more than eight weeks. More than four out of ten complaints were upheld in favour of the customer.

The bank said it was going through a period of growth and business integration which had caused issues for some customers. Branch and call centre staff had been given more support and a guide to the main matters of complaint along with immediate access to internal experts to help sort out issues.

Steve Williams, director of service quality and complaints, said: “Whilst we are pleased that the volume of complaints received has decreased from the same time last year, we know we need to do more.

“Improving service quality remains a priority for Santander, as we don’t want any customer to be disappointed by the experience they receive from banking with us.”

Meanwhile, Barclays is planning to spend £1 billion on boosting customer service across its global retail banking business by 2013.

The bank is to spend £250 million a year on improvements including branch overhauls in the UK as well as investing more in technology such as mobile banking.

Barclays has retail banking operations in 21 markets, although 39 per cent of its £1.8 billion global profits from the division last year were generated in the UK, where 15 million of its 37 million customers are based.

The bank, which has revamped around half of its 1,700 UK branches so far, has seen growth in current account share more than double at modernised branches compared to those areas still awaiting investment.

The group also plans to add up to four million banking customers by 2013 with expansion in Spain, Portugal and affluent cities in northern Italy.