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Deloitte, PwC and KPMG have launched a scathing attack on the Internal Revenue Service, which they accused of “a pattern of arbitrary, capricious and unreasonable conduct” towards multinational companies that risks eroding confidence in the US tax system.

The three accounting firms made the claims about the US tax authority in a court filing this week supporting Coca-Cola’s attempt to overturn a ruling by the agency that could cost the drinks maker $18bn.

The dispute centres on “transfer pricing” arrangements relating to the allocation of profits between Coca-Cola subsidiaries in different countries. The tax arrangements had been blessed by the other Big Four accountancy, EY, which has audited the drinks maker’s financial statements since 1921.

In a joint submission to the Atlanta appeals court, Deloitte, PwC and KPMG backed Coke’s argument that the IRS had acted unfairly by changing the tax treatment of intercompany payments involving subsidiaries making syrups for its fizzy drinks.

The firms, which together advise the majority of US multinationals on their taxes, said the IRS was failing to live up to its stated mission to help taxpayers “understand and meet their tax responsibilities”.

The firms, which are not parties in the litigation, wrote they “hold a deeper concern” that the tax authority’s actions in the case “are part of a broader pattern of IRS enforcement actions that have whipsawed taxpayers across the country”.

They added: “In our collective experience, the IRS in recent years has routinely felt empowered to discard and disavow prior agreements and audit history without supplying a justification for the proposed change.”

The firms cited other cases involving the medical devices maker Medtronic and the industrial group Eaton Corp, adding that upholding the IRS’s decisions in the Coca-Cola case “would risk significant negative consequences for the US tax system”.

The accounting firms said taxpayers relied on the IRS adopting consistent policies in its approach to transfer pricing. “If this court does not restrain the IRS’s unjustified shifts in position, other taxpayers will likely be subjected to similar arbitrary and unreasonable behaviour, contributing to an erosion of confidence in the fairness of the US tax system,” they added.

The IRS said it does not comment on pending litigation.

The dispute with Coke is one of the most financially significant being fought by the US tax authorities and is being widely followed in corporate America. Business lobby groups including the US Chamber of Commerce and the National Association of Manufacturers have weighed in to support Coke.

The company has paid $6bn after losing the first round of its legal battle against the IRS, covering the tax years 2007 to 2009, but has told shareholders it is confident of winning its appeal and getting the money back.

If it loses, and the IRS’s new treatment of Coke’s transfer pricing arrangements is vindicated, the company estimates it would have to pay a further $12bn to cover the shortfall since 2009 and result in it paying a higher rate in future.

The tax treatment of payments involving Coke’s syrup makers, which are typically located in low-tax jurisdictions, has been a running sore between Coke and the IRS for decades.

A similar dispute was settled in 1996 by reallocating more profits to the US parent company. Coke used the formula agreed in that case to calculate its tax returns for another decade without objection, before the IRS decided in 2015 that it had improperly suppressed US profits and ordered it to pay billions of dollars of back taxes.

The company is arguing this U-turn was “arbitrary and capricious”.

“If allowed to stand,” Deloitte, PwC and KPMG wrote in the their supporting brief, “it could have a deleterious effect on tax administration by needlessly creating controversies rather than fostering a co-operative environment among taxpayers and the IRS.”