Lloyds, Bank of Scotland, MBNA and Santander made up the rest of the top five.

The figures show that there were 3,422,384 complaints over the period, an increase of 1% on the first half of 2012.

The top five complained about financial products, in terms of number of complaints opened in the second half of 2012, are:

• Payment protection insurance (PPI) – 2,170,537 (a 5% increase on the first half of 2012)

• Current accounts – 304,196 (a 6% decrease on the first half of 2012)

• Other general insurance products (excluding PPI) such as travel, motor or critical illness insurance – 296,679 (a 6% increase on the first half of 2012)

• Credit cards – 283,705 (a 14% decrease on the first half of 2012)

• Savings (including cash ISAs) and other banking products – 100,797 (a 20% decrease on the first half of 2012)

Santander UK topped the list of most complained about in the banking sector, attracting 125,451 complaints during the period.

It was followed closely by Barclays, which attracted 118,031 banking complaints.

Santander and Barclays also saw most complaints in the investments sector, with Santander attracting 2,236, just above Barclays with 2,151.

For the decumulation, life and pensions sector the Prudential Assurance Company Limited was the most complained about, with 7,788.

It was followed by Aviva with 6,554 complaints.

Martin Wheatley, FCA chief executive, said: “Greater transparency drives greater competition, and the publication of the complaints data lays bare the track record of the UK’s financial institutions when it comes to resolving customer conflicts.

“When I meet with the bosses of the financial institutions they frequently tell me that they don’t want to be at the top of the table, which means they strive to improve both their sales and complaints handling processes. Not only does our data help consumers compare and contrast their current bank or lender, but it also boosts competition among firms too.”