The High Court has ruled in favour of the government in a judicial review of the measures to tackle online copyright infringement in the Digital Economy Act (DEA).
BT and TalkTalk prompted the judicial review in November 2010 by claiming that the act was not enforceable under EU law.
The government has been given the green light to go ahead with its plans to monitor ISP networks and issue warning letters to alleged illegal downloaders.
"We're disappointed that we were unsuccessful on most of the Judicial Review," said a TalkTalk spokesperson.
"We are reviewing this long and complex judgement and considering our options, which may include an appeal to the UK Court of Appeal, or a request that the Court of Appeal make a reference to the European Court of Justice," they added.
"Though we may have lost this particular battle, we will continue fighting to defend our customers' rights against this ill-judged legislation."
BT and TalkTalk were also concerned that some of the financial burden lay with them to alert users accessing illegal sites.
This is the one area in which the ISPs had some success, as the judge ruled that they should not be forced to pay a share of the cost in setting up, monitoring and enforcing the mass notification system.
However, ISPs will still have to pay a share of the operating costs.
Chief executive of the British Phonographic Industry (BPI), Geoff Taylor, has said he is extremely pleased with the result, and suggested BT and TalkTalk should not consider any further action.
"This confirms that the DEA is proportionate and consistent with European Law," said Taylor.
"Shareholders and customers of BT and TalkTalk might ask why so much time and money has been spent challenging an Act of Parliament to help reduce the illegal traffic on their networks," he added.
"It is now time for BT and TalkTalk to work constructively with government and rights holders to implement the Digital Economy Act".
The BPI has been working closely with ISPs and the government to establish a ‘Plan B' on how website blocking for those infringing copyright could work effectively.
However, digital rights campaigners the Open Rights Group (ORG) have spoken out in support of BT and TalkTalk, and hope that they will take further legal action.
"It is important to remember that this is not a judgement on whether the Digital Economy Act is good public policy," said Peter Bradwell, campaigner at the ORG.
"We still believe that, if enacted, the Act will hurt people's privacy and access to the internet for no proven gain. We hope that BT and TalkTalk will appeal and we will support them if they do."
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