A suggestion that rail fare and wage increases should be linked to a lower inflation measure has sparked an angry response from unions.

Transport Secretary Chris Grayling wants future increases to be based on the Consumer Price Index, rather than the current, higher Retail Price Index.

The RMT union accused him of trying to impose a "pay cap" on its members.

It comes on the day that the government publishes the latest RPI figure, which will decide rail fares from January.

Under the current system, July's RPI sets the maximum rise for regulated fares in January.

That figure, which will be announced at 09:30 BST, is expected to be about 3.5%.

Mr Grayling said he wanted to see "lower levels of increases for passengers in future".

He also said that, if a lower measure of inflation is used to calculate ticket price increases, then it should also be used for costs, including annual rises to workers' salaries.

In response, RMT general secretary Mick Cash said: "If Chris Grayling seriously thinks that front line rail workers are going to pay the price for his gross incompetence and the greed of the private train companies he's got another thing coming.

"This is a basket case government and a lame duck transport secretary continuing all-out war on staff and passengers alike.

Protests planned
"RMT will fight any attempt to impose a pay cap on our members in a drive to protect private train company profits."

Meanwhile, RMT is organising protests on Wednesday outside stations in London, Birmingham, Cardiff, Leeds and Edinburgh against fare rises.

The union claims passengers are paying "through the nose" for overcrowded services.

Research found the cost of rail travel has increased more than twice as fast as wages since 2008.

The TUC said fares have risen by 42% over the past 10 years, while nominal weekly earnings have only grown by 18%.