A US federal judge has blocked the implementation of an anti-telemarketing list which would have prevented salespeople telephoning Americans to solicit business.
It is likely the case will end up in the Supreme Court
The ruling comes only hours after US Congress on Thursday passed legislation granting the Federal Trade Commission power to enforce the so-called "Do Not Call" list, which more than 50 million Americans have subscribed to.
US District Judge Edward Nottingham in Denver, Colorado, said the list violated constitutional free-speech protections as it amounted to "a government restriction on lawful and truthful commercial speech".
The ruling is a victory in part for telemarketers, who claim the list infringes on free speech rights and will devastate their industry, causing thousands of job losses.
Unless the ruling is overturned by a higher court, the decision could prevent the US Government from pressing ahead with plans to launch the register at the beginning of October.
The FTC is appealing the ruling and it is likely the case will end up in the nation's Supreme Court.
Both Democrat and Republican politicians sides condemned the decision as contrary to the wishes of Americans.
Republican senator John McCain described the judge's move as "misguided," while a Democratic senator said: "It's not an infringement on telemarketers' speech if I
don't want to take their calls".
The FTC hopes the Do Not Call registry list will block 80% of all calls made by telemarketers.
Exemptions include calls from charities, pollsters and political campaigns.
Under the rules, telemarketers will have to check the list every three months to see who does not wish to be called and those who call listed people could be fined up to $11,000 each time.
Thursday's ruling was the second barrier to the US Federal Trade Commission's attempts to push the legislation through, after another federal judge on Tuesday in Oklahoma City, Oklahoma ruled the agency lacked the power to create and operate the registry.
It was the Tuesday ruling that prompted Congress to act in such an unusually speedy manner, writing and pushing through the bill in only two days.
US President George W Bush has also indicated he will sign it into law.
Analysts say that Congress could possibly fix the problem by extending the list to cover calls excluded in earlier bills.
But telemarketers have vowed to push on with more lawsuits in other states to protect their livelihood.