Simple announced plans to move all of its customer accounts to BBVA Compass in April last year, becoming the first user of the Spanish bank's API in the United States.

The initiative came just a year after BBVA acquired Simple and its 100,000 customer accounts for $117 million, effectively severing ties with former banking partner The Bancorp Bank. But with time running out on the transitional arrangement, Simple has written to a number of its customers with "some difficult news".

"We have to close your Simple account on May 15 because we're not able to move it to our new partner bank, BBVA Compass."

Acknowledging that the news would be "jarring" to those customers unaware of the back-end transition, Simple went on to explain: "Due to logistical issues on our end, we weren't able to start moving your account in time. Now that we're appraching the end of our relationship with our current partner bank, we have to close all the accounts we haven't moved yet - including yours."

The bank is advising affected customers to cancel all automated payments in and out of their accounts and withdraw all deposits ahead of the shut down.

"If you can look past our clumsiness with this move and you'd still like to bank with us, we'd love to have you," the letter continues. "though we can't keep this account open, you can apply for a new one as soon as we've finished the closure process."

In February, BBVA Compass took a $60 million goodwill impairment charge against Simple in its fourth quarter results statement, although the group remained upbeat, claiming that Simple was signing up more than 30,000 customers a month.