Lloyds Bank shed 20,065 current account customers in the three months after the arrival of the seven-day switching guarantee in late September.
It lost 103,704 customers between October and December 2013 but could only attract 83,639, according to data from the Payments Council.
Scandal-hit bank The Co-operative also experienced a net decline in current account customers during the period, of 15,231.
NatWest and Barclays only managed to hold on to a few more customers, with net losses of 14,312 and 13,379 respectively.
At the other end of the spectrum, the current account provider that was able to attract the most new customers was Halifax, which celebrated the arrival of 66,401 new customers and only lost 20,069, making for a net gain of 46,332.
Somewhat ironically, Halifax also belongs to the same group as Lloyds Bank, giving parent Lloyds Banking Group both the winner and the loser in the switching stakes.
Philip Robinson, director of current accounts at Lloyds Bank, said: "The figures published for Lloyds Bank include total switches for two brands that were operating separately throughout Q4 2013, and are therefore not representative of past or current switching activity for Lloyds Bank.
"Since the launch of the current account switching service we have revitalised Lloyds Bank and launched our innovative Club Lloyds current account, which pays up to 4% credit interest, alongside the new Everyday Offers and It’s on Us programmes. We will continue to deliver good value products with innovative features, backed up with a high standard of customer service."
Santander was another big winner, following just behind Halifax with a net gain of 30,001 customers as people flocked to its popular 123 account. Nationwide took third spot, with a net gain of 12,679 customers.
Overall, during the three-month period, current account switching increased by 17% as consumers felt confident enough to "break free from the shackles of their old banks".
Figures from the switching website from January revealed that its users had expressed most interest in the First Direct 1st Account and the Halifax Reward account.
The latest data from the Payments Council showed that since the introduction of the seven-day current account switch guarantee, more than 898, 935 people had switched accounts by June 2014.
Net customer losses/gains between 1 October and 31 December 2013
Lloyds Bank -20,065
Clydesdale Bank -6,787
Ulster Bank -1,071
AIB Group (UK) -979
Bank Of Ireland (UK) -379
Bank of Scotland -97