Uber is facing a backlash from its drivers after changing its pay structure in 20 cities across the US.

Drivers in San Francisco and San Diego, where the change was implemented in the last two weeks, say that their earnings have fallen as a result of changes to how their pay is calculated.

The news comes as the company reportedly filed confidential paperwork for an initial public offering last week with the US Securities and Exchange Commission. A spokesperson for the company declined to comment.

The ride sharing app began to change the formula for how it pays drivers in cities across the US in July.

Uber pays its drivers based on a formula consisting of separate base fare, distance and time rates. The company has re-calculated the formula for the first time since the app was launched so that greater weight is given to time and less is given to the distance travelled.

The new formula means that drivers will make less from long journeys and more from slower, shorter rides.

In San Francisco, for example, an Uber driver has gone from making $1.33 a mile to 91 cents a mile. At the same time, Uber has raised the payment for time spent on each ride from 24 cents a minute to 39 cents a minute. Rates in the surrounding Bay Area, including Oakland and South Bay, differ.