Vodafone had a section of its website hijacked on Thursday, as blogs were posted claiming the communications giant owed billions in tax.
UK Uncut managed to gain a password to Vodafone's World of Difference website, posting claims the firm owed 6 billion to the UK Government.
The campaign group called on the Government to take action, suggesting the funds could be used to support charities, which are having their budgets slashed.
The Vodafone World of Difference programme hands grants to people wanting to carry out charity work. One recipient, who blogged on the website, leaked their password to UK Uncut.
Photos and videos of UK Uncut's protests at Vodafone stores were posted alongside the blogs.
"Vodafone make a big deal out of their funding to charity, but their charitable giving pales into insignificance compared to the 6 billion they got away without paying in tax," one of the grant winners said.
"I work for a charity and understand how the government's brutal spending cuts will hit the poorest and most vulnerable."
Vodafone said it has no unpaid tax bill and HM Revenue & Customs (HMRC) has labelled the 6 billion figure being used by protesters as an "urban myth."
The firm also noted it contributes over 700 million every year to the Exchequer.
However, last year the firm did agree to pay 1.25 billion in a tax settlement with HMRC.
The dispute was over controlled foreign companies tax and the case ended with the largest corporate tax settlement in UK history.
The communications giant also stressed the website was not hacked, as a number of reports had suggested.
Vodafone said there was no risk to World of Difference winners or to customers' personal data.
"The posts which appeared on two World of Difference winners' blogs were published as a result of an individual sharing log in details with a protest group, not as a result of a 'hack,'" a Vodafone spokesperson said.
"World of Difference winners are doing great things for charities up and down the country. It's very sad to see how low people will go to further spread misinformation and for the charitable programme to be used as a platform for this kind of protest."
