A study by consultants Simon-Kucher and Partners has found there are differences in customer loyalty between the UK’s largest banks when purchasing additional products.
According to the study, almost a third (30%) of Nationwide current account customers have an ISA.
In second place was Lloyds Banking Group (25%), followed by RBS Group (21%), HSBC (21%), Santander (16%), Barclays (16%) and Co-operative Bank (11%).
Jens Baumgarten, banking partner of Simon Kucher and Partners, said: "When it comes to ISAs, banks must act smartly and quickly to secure the business of their customers.
"The pricing of ISAs is the key determinant for banks to be able to attract and retain customers."
The study also examined overall customer loyalty for all banking products.
Nationwide customers proved to be the most loyal with an average of 1.31 products held in addition to their current account.
In second place was Lloyds Banking Group (1.24), followed by Barclays (1.14), RBS Group (1.13), HSBC (1.08), Santander (1.02) and Co-operative Bank (0.76).
Ben Snowman, director and author of the study said: "Current accounts are known as the anchor product in the relationship between customer and bank.
"However, there are clearly vast differences in the loyalty that current account customers exhibit between the top seven high street brands."
Snowman said the ISA market is set to change as a result of the seven day switching scheme that will be introduced in September 2013.
According to previous research by Simon-Kucher, account switching rates were likely to increase from 2.5-5% (2012) to 6-12% as a result of account switching.
More than one third (37%) of current account holders would also consider switching to a ‘challenger’ brand, where Virgin Money is the most popular.
Snowman said: "These two effects will give customers more choice in selecting a current account provider, combined with greater ease of access to alternative banking brands.
"Banks must secure the loyalty of their customers now, or risk them looking elsewhere for ISAs and other banking products."