This article originally appeared on Forbes.com.

The UK's five largest banks, which together control 80% of the country's retail banking market, face big challenges—none bigger than the new open banking regulations that took effect in January.

The new rules, mandated by the UK's Competition and Markets Authority and closely watched by bankers and regulators in the US, aim to foster competitive markets and encourage innovation. The regulations require banks to disclose performance and fee data, which will make it easier for customers to compare the offerings and results of different financial providers.

The new rules also direct banks to use open application programming interfaces (APIs). With open APIs, customers can readily share their financial information with other providers, if they choose to do so. Open APIs will also make it much easier for customers of big banks to transfer their accounts, manage payments, and conduct transactions through other banks and nonbanks.

Open banking is good news for consumers. With open APIs, many of the long-standing barriers to switching providers will dissipate. Big banks face the prospect that many of their customers may seek out the convenience of digital aggregators, taking their accounts, and the profit pools they represent, with them.

As much as 10% to 20% of banking profits could be at risk of disruption. Bain & Company estimates that within five years, £1 billion to £2 billion of annual pretax profits could be vulnerable to disintermediation.

A survey of more than 4,000 UK banking customers conducted by Bain, Salesforce and MaritzCX shows that big banks have reason to be concerned about open banking. Their most important customers—those who have a primary banking relationship—are at risk of leaving: 63% say they are willing to share financial information concerning their accounts with a competing bank, fintech or aggregator in pursuit of a better offer.

Our survey shows that certain customers are "high risk"—that is, more open to appeals from disruptors in an open banking environment. Those most likely to leave for another provider tend to younger and more affluent—age 55 or younger with an annual household income of at least £55,000. Additionally, the majority of these consumers have already adopted at least one alternative fintech solution, such as Apple Pay. Among these high-risk customers, 65% are open to sharing their personal data with other banks or nonbanks in order to get better products or services. While high-risk customers account for only 15% to 20% of the total customer base, we estimate that they make up close to 45% of bank profits.

Among all bank customers surveyed, 59% report already being willing to try a platform that aggregates banking products and other services (such as insurance and investment advice) in order to save money and simplify financial transactions. Within the high-risk group, that figure is much higher, at 74%.

Recognizing these vulnerabilities, major UK banks are taking steps to cope with the disruption that open banking is bringing.

Incumbents have at least one clear advantage: They already have access to a sizeable pool of customers. By keenly paying attention to their customers, they can gain insights into the products and experiences they want most, now and in the future. Banks can draw on a wealth of customer data from banking apps, cash flow, credit history, mobile location and browser histories.

Big banks also have an edge when it comes to trust. When we asked customers if they are willing to share more data with a provider in order to get a better product offering, 78% said they are willing to share with their primary bank, while 63% said they would share with another bank, and just 43% would share with a nonbank.

Forward-looking banks are confronting the aggregator challenge head-on. They are adapting their existing platforms so that they can easily aggregate data from external sources; they are setting up distribution partnerships with third-party platforms; and they are investing in new data and service providers, such as digital brokers, to move further up the value chain.

Banks entering the open banking era are developing the muscles they need to change and adapt quickly. They are organizing themselves around customer needs in a way that enables faster innovation cycles and eliminates silos that get in the way of decision-making. Change-ready banks are also taking a hard look at ways to simplify and automate back-end processes, remove legacy costs, eliminate options that customers no longer value, and deliver experiences that are 100% digital.

With the arrival of open banking, UK consumers are primed for change and willing to experiment with new ways to manage their money, borrow, and protect their wealth. Industry leaders who act now to redefine their strategies around key customers segments have an opportunity to get ahead of the curve. Those who don't act now run the risk of being left behind by their own customers.

Stanford Swinton and Eduardo Roma are partners in Bain & Company's Financial Services practice and based in London.