TalkTalk have revealed the cyber attack cost the company up to £35m.
The company confirmed to ITV News that up to 21,000 customers bank account details were held on the site unencrypted and that they were held on parts of the website targeted by suspected hackers.
It said "fewer than" 1.2 million email addresses, names and phone numbers were accessed in the attack.
Announcing its results for the first six months of 2015 today, the company said the cyber attack had cost the company between £30m and £35m.
It also announced it was offering all its customers, not just those affected by the hack, the choice of a free upgrade in the wake of the attack.
They will be able to add one of the following to their existing services with the group: TV content including movies, kids entertainment and sports; a mobile SIM with a monthly allowance of free texts, data and calls; unlimited UK landline and mobile calls; or a broadband health check by experienced engineers.
The group has also created a new bundle of online and telephone security features to boost customer protection.