Funky Pigeon has suspended all orders from its website following a "cyber security incident".
The online cards and gifts retailer, which is owned by WH Smith, said it was targeted by hackers last Thursday and has been forced to take its system offline as a precaution.
The company is investigating whether any personal data, including names, addresses, email addresses and personalised card designs, have been accessed.
But it said no payment information or customer account passwords were accessed during the hack.
Funky Pigeon said it will write to all customers from the past 12 months to inform them of the cyber attack.
You should look out for an email from Funky Pigeon to let you know if there are any precautions you need to take to protect your data.
Parent company WH Smith said in a statement: “We take the security of customer data extremely seriously.
"The company has temporarily suspended orders from the website and is currently investigating the detail of the incident with external IT specialists.
"No customer payment data, such as bank account or credit card details, has been placed at risk - all of this data is processed securely via accredited third-parties and is securely encrypted.”
It added: “We would like to sincerely apologise to our customers for any concern or disruption this may cause, and reassure them that our teams are working around the clock to investigate and resolve this incident.
“As our investigation progresses, we will provide further updates to customers and other affected parties as necessary.”
Funky Pigeon says it has informed relevant regulators and law enforcement authorities about the security breach.
It marks the second attack on a retailer in recent weeks following a similar incident at The Works.
The arts and crafts chain had to shut down access to its data internally and externally earlier this month after its systems were targeted by hackers.
This resulted in new stock deliveries to its shops being temporarily suspended and longer delivery times for online orders.
The Works trades online and from more than 500 stores across the UK and Ireland.
Google has created a free add-on that allows Chrome users to check if their online credentials have been stolen.
Password Checkup was first released in 2019 as a free extension and it's designed to strengthen online security.
By scanning known databases, it pinpoints usernames and passwords which hackers have lifted from websites and made available to cyber criminals.
Password Checkup urges users to switch up their access information if it has been leaked.
