A number of customers of the mobile-only banking service Monzo have threatened to leave after finding out the company's policy on transgender employees.
The bank tweeted that "trans women are women" and "trans men are men" after Twitter user, Abigail McGyn, criticised their pledge to increase the number of senior staff who "identify as women".
In a tweet to the company, Ms McGyn said: "I'm concerned that Monzo's Head of People has suggested that 50 per cent men & 50 per cent transwomen would achieve your objective.
"How is zero female people inclusive or diverse?".
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Following the Twitter exchange, a number of Monzo users threatened to abandon the service.
One user wrote: "Losing a customer here. Transwomen are transwomen and enjoy all the human rights offered to society. They are not women and it is appalling that our sex-based rights are being trampled on".
Another tweeted the hashtag: "#boycottmonzo".
Monzo is currently signed up to the government's Women in Finance Charter, which aims to increase the number of women in senior roles within the banking industry.
Ms McGyn told talkRADIO that she thought the company signing up to the Charter whilst allowing staff to self-ID their gender was an "insult".
"I actually agree with companies increasing trans representation, but it should not be instead of increasing females. The two groups have different needs and are discriminated against for different reasons," she said.
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"Maternity discrimination is a key factor in why women get left behind in finance, and that only impacts people who are female, and puts transwomen and females in conflict.
"Saying 'transwomen are women' might be kind, but males who self-identify as women are not biologically, or legally, women. Stating that they are is an ideological position and is being used as a front to allow continued discrimination against females -a protected category."
Defending its policy, Monzo told talkRADIO that the company stands by transgender employees and "their right to self-identification".
"Our mission is to make money work for everyone, and that means supporting inclusion - both amongst our staff and for people who use Monzo," a spokesperson said.
"Monzo is built by people from a range of backgrounds, including trans employees. We stand by them, and their right to self-identification. And we don't agree that this is incompatible with our commitments to diversity."