US companies will have to disclose the gap between their chief executives’ pay and an average employee’s salary under a new rule that will provide fresh data on the politically charged issue of inequality.

The Securities and Exchange Commission, the US market regulator, voted by 3-2 yesterday to adopt a rule that requires listed companies to disclose the pay ratio between chief executives and their median employee.

Mandated by the post-crisis Dodd-Frank reforms, the rule is supported by unions and Democrats worried about persistent income inequality, but opposed by companies that call it a costly and time-consuming gimmick.

The ratio’s practical utility to investors is open to question.

Mary Jo White, the SEC’s chair, said the rule “will provide shareholders with additional . . . information that they can use when considering a company’s executive compensation practices, an important area of corporate governance on which shareholders now have an advisory vote”.

But Regina Olshan, an executive compensation expert at law firm Skadden Arps, said: “I don’t know any investor who has any interest in this.” She added that investors were more interested in how chief executive pay compared to that of their deputies.

The ratios will nonetheless become political fodder as US presidential candidates from both parties have pointed to inequality and middle-class wage stagnation as one of the most pressing issues in the 2016 election.

The AFL-CIO, the US’s biggest union federation, played a leading role in lobbying for the rule and has said it should “shame” companies into lowering chief executive pay.

John Engler, president of the Business Roundtable, a lobby group of chief executives, said: “It’s costly, meaningless, and all it does is create compliance risks for business and talking points for people trying to score political points.”

Despite the ostensible simplicity of working out a median wage, he said the task was complicated by part-time employees, seasonal workers and staff in foreign countries with different forms of social security contributions.

Asked if business groups would file lawsuits against the rule, Mr Engler said it was not clear what their basis would be. “It is constitutional for Congress to enact dumb laws. They’ve done it before. And this is one of the dumber ones.”

Richard Trumka, AFL-CIO president, said in a CNN op-ed that complaints about the difficulty of implementing the rule were nonsense. “Employers should already have this information on the books. Dodd-Frank asks companies to do some simple calculations, not put a man on Mars.”

He noted that Walmart’s chief executive Doug McMillon earned $19.4m in 2014 — although that included performance-related stock grants not yet vested — while the retailers’ typical employee starts on $9 an hour.

“American consumers and workers . . . have a right to accountability and transparency when it comes to the pay practices of corporations,” Mr Trumka wrote. “Wage and income inequality has spiralled out of control.”

In response to criticism of a draft proposal, the SEC has exempted some companies from the rule — including start-ups and foreign businesses — and granted flexibility to others to choose how they select the median employee used to calculate the ratio.

But the changes did not placate the SEC’s two Republican commissioners, who voted against the rule.

Daniel Gallagher, one of them, said: “Addressing perceived income inequality is not the province of the securities law or the commission. And yet here we are . . . adopting a nakedly political rule that hijacks the SEC’s disclosure regime to once again effect social change desired by ideologues and special interest groups.”

The US Chamber of Commerce, the biggest business lobby, has fought against the rule and warned it will cost corporate America more than $700m to implement.