BT admitted it could do little to stop web users disabling its sites after three flagship domains were knocked out on Friday.
bt.com, btinternet.com and gameplay.com were all put out of use after they were flooded with requests.
A BT spokesman said the sites were overwhelmed rather than hacked and no confidential areas were accessed.
The Register website said it received an e-mail claiming credit for the attack.
It gave no name but read: "This is my payback to BT for ripping this country off."
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