The official website said there was no evidence of an attack but that security was affected on some transactions carried out between August 29 and September 5.
Credit and debit card details were not affected but other personal information including names, addresses, emails and bank account numbers and sort codes may not have been properly encrypted.
A statement on TV Licensing’s website said: ‘We were recently alerted to an issue with our website’s security following a technical update. We took the site down straight away so that we could fix it.
‘We take the security of our customer’s data very seriously. That’s why it’s our normal practice that when our customers make payments or send us financial or other personal details through our website, the data is encrypted to keep it safe.
‘While there is no evidence that our website has been subject to any sort of attack or that the security of our customers’ data has been compromised, we recently discovered that for a limited period – from August 29 until around 3.20pm on September 2018 5 – some transactions carried out on the website were not as secure as they should have been.
‘This issue did not affect debit and credit card details but it may have affected customers’ personal details such as name, address and email or, if customers entered bank details, the sort code and account number.
‘In some cases, this information was not encrypted when it was transmitted from the customer’s computer.’
A statement added: ‘We’re really sorry this happened but want to reassure customers that the risk is low and we’ve taken action to ensure it doesn’t happen again. There is no evidence of any attack and we’ve found no evidence of any unauthorised access to information.
‘We’ve published further information on our website which can be found at www.tvlicensing.co.uk/secure.’
The website was taken offline after the issue was discovered and TV Licensing said the risk of customers being affected was ‘low’.
The firm urged anyone concerned to check their bank accounts for any unauthorised transactions and contact their banks should they find anything suspicious.
It added it had contacted any customers believed to have entered their bank details during the affected period.
The statement added: ‘As soon as we discovered this issue, we took the website offline to urgently fix it.
‘The website has been restored with full security and all transactions are now encrypted again under HTTPS.
‘We’ve urgently investigated and have established the cause so we can ensure it doesn’t happen again.
‘Additionally, we’re contacting all customers urgently who we believe submitted their bank details during this period.’
TV Licensing added anyone who suffers a financial loss would be compensated.
Anyone who believes they were affected can contact TV Licensing on 0300 790 6035.