Starbucks could face angry protests on Saturday after pressure group UK Uncut said it planned to start targeting the coffee chain's stores.

The group, which has previously organised sit-ins at Vodafone shops and at upmarket grocer Fortnum & Mason, will be meeting outside St Paul's in London on Saturday morning under the banner "no cuts, no tax-dodging", before joining a TUC march through central London against the government's austerity programme.

However, Starbucks said it has no plans to close any of its central London locations, including the branch directly opposite where the protesters will be meeting.

At the same time as campaigners were planning protests, train operator CrossCountry said it might stop selling Starbucks coffee on its services, and a new poll found that the coffee firm's brand identity had taken a "disastrous" hit from the revelations that has paid just £8.6m in tax since launching in the UK 14 years ago.

Bosses at the company are thought to have been in contact with HM Revenue and Customs to clarify its tax position. On Thursday, HMRC estimated that, under its definition of what constitutes tax avoidance, £5bn had been lost last year — the same amount as for 2009-10.

UK Uncut, which organised a sit-in at Vodafone's most high-profile London store in 2010, had always planned to join the TUC march to protest against government cuts

Anna Walker, a spokesperson for UK Uncut, said: "Companies such as Starbucks are definitely targets in our sights for future protests. UK Uncut's previous sit-ins and occupations in the branches of tax dodgers have proved very effective in highlighting the unjust practices of big business." The organisation last hit the headlines in the summer after activists attempted to storm the offices of healthcare assessor company Atos.

A spokesperson for Starbucks said: "Obviously, the safety of our staff is paramount, but we currently have no plans to close stores in central London over the weekend."

Starbucks has rung up total sales of £3bn in the UK since opening its first store in 1998, but has recorded a loss for every year since it started trading in Britain, despite US management claiming it had been a "profitable" marketplace.

On Thursday, YouGov's BrandIndex, which records the strength of companies' brand identity, revealed Starbucks has plummeted in the past few days. Its "buzz" score, which measures the number of negative and positive comments customers have heard, reached a four-year low, falling to -13.9 from +0.7. Its reputation score has also fallen from 4.6 a week ago to -3.9 on Thursday and could continue to drop.

Sarah Murphy of BrandIndex said: "To say this story has been a disaster for the Starbucks brand would be a bit of an understatement. It's still too early to say what the long-term impact of this is going to be, but in the current climate we've seen the public take a fairly dim view towards accusations of corporate greed."

CrossCountry, which operates between Aberdeen and Penzance, said: "If our customers were to tell us they were unhappy for us to sell them Starbucks coffee then we would, of course, review this arrangement."

Starbucks UK's managing director, Kris Engskov, tried to justify the company's position in a blog post on the company's website. In an initial version he wrote: "The most important thing to understand is that Starbucks does pay tax in the UK. Indeed over the last three years we have paid over £160m in various taxes, including Pay As You Earn for our 8,500 UK employees, national insurance and business rates."

The post was subsequently changed to remove the reference to PAYE, which is a tax on employees' earnings, not on businesses, but the figure of £160m remained. The company said £87m of that was VAT – which is a tax on consumers.

Several MPs have called for an inquiry into Starbucks' tax affairs, which are considered perfectly legal, and for HMRC to answer questions over its relationship with the company.

• This article was corrected on 2 December to include only the figure for the HMRC definition of tax avoidance. The earlier version referred to the total figure for the tax gap.