Starbucks is volunteering to pay £10m in taxes in each of the next two years as it attempts to win back customers following revelations that it has paid no corporation tax in the UK in the past three years.

Kris Engskov, managing director of Starbucks UK, said he was surprised by the public outcry at the company's use of complex tax arrangements and vowed to make the payments to HM Revenue and Customs for two years even if the coffee chain failed to make a profit.

However, politicians and tax experts said the unilateral decision, which Engskov said he had not discussed with HMRC, made a mockery of the tax system, while the tax authorities reaffirmed the position that corporation tax is not voluntary.

The unprecedented announcement was made at the London Chamber of Commerce, where Engskov said: "I am announcing changes which will results in Starbucks paying higher corporation tax in the UK – above what is currently required by law.

"Specifically, in 2013 and 2014 Starbucks will not claim tax deductions for royalties or payments related to our intercompany charges.

"In addition, we are making a commitment that we will propose to pay a significant amount of corporation tax during 2013 and 2014 regardless of whether our company is profitable during these years.

"These decisions are the right things for us to do. We've heard that loud and clear from our customers."

Although he said the tax authorities were unaware of the company's plans, he added they had been in negotiations generally.

Corporation tax is levied on profits but Starbucks has avoided paying it for three years in the UK through complex international payments within the company known as transfer pricing. Starbucks currently makes a loss due to a 4.7% premium paid to the Netherlands division – where the coffee beans are roasted, and another 20% premium to Switzerland to buy the coffee beans. The company said it would not claim deductions on these payments, or against intercompany loans.

Chris Morgan, head of tax policy at KPMG, said: "I think what they've said is absolutely extraordinary and really does change the tax landscape. It is the first time I have seen public opinion make a company change its mind.

"This is clearly a voluntary decision despite taxes being collected according to law and principles. HMRC might be very nervous about how things are done because their job is to collect the right amount of tax and they don't want to overcharge people."

Conor Delaney, tax lawyer at Milestone International Tax Partners, said it "made a mockery" of the tax system.

He said: "You have a fundamental principle that you can only be taxed by clear legislation and yet you have this process where a company is hauled up and publicly embarrassed and blackmailed into volunteering more tax."

Other multinationals including Google and Amazon have come under attack for their use of similar tax arrangements, which involve funnelling cash through European subsidiaries based where tax levels are lower.

Last night, Google and Amazon declined to follow Starbucks' lead, insisting they pay the correct level of tax.

The Liberal Democrat peer Lord Oakeshott said: "It really makes me sick to see Starbucks taking the credit for a voluntary decision to pay some tax just because consumer resistance and publicity has hit them where it hurts – in the pocket.

"When will HMRC get a proper grip of transfer pricing and royalty ruses that companies like Starbucks use to conceal their real profitability in this country?"

Conservative MP Steve Barclay, who sits on the public accounts committee, which grilled bosses from the three companies last month, said he was pleased with the decision.

However, he warned: "This payment is more to do with corporate reputation than corporation tax.

"Companies have a duty to shareholders to maximise their profits, so it is difficult for Starbucks to argue that they are making this payment on tax grounds, rather than to protect their brand identity."

Engskov continued to insist that the company's business in the UK remained unprofitable despite contradictory statements by board members in investor presentations at the company's US headquarters.

Starbucks UK would need to turn a profit of £41.6m if it was to legitimately owe HMRC the £10m corporation tax it plans to pay next year.

However, it has made a loss for nearly every year it has traded in the UK, paying just £8.6m in corporation tax over the 14 years it has been here due to challenges by HMRC.

UK Uncut, which is encouraging direct action against the chain, was not appeased. Spokesperson Hannah Pearce said: "The £10m that Starbucks have estimated they may end up paying is £5m less than that paid by their nearest competitor, Costa Coffee.

"This weekend 40 actions will take place in Starbucks stores in towns and cities across the country. People will be transforming Starbucks stores into refuges, creches and other services which the government are cutting with their unjust and unnecessary austerity plans," she said.

HMRC declined to speak about individual companies' tax affair, but said: "Corporation Tax is not a voluntary tax and Parliament sets out the rules and rates for businesses to follow.

"The public expects businesses to pay their fair share and HMRC will challenge, through the courts if necessary, any structures or tax payments that do not comply with the UK tax law."

• This story was altered on 6 December 2012. The original gave the impression that Starbucks would pay £10m in tax over two years - in fact, it will pay £10m a year for two years.