He was responding to the Office for National Statistics' latest monthly inflation figures.
These showed that the rate of the retail price index (RPI) which is used as the basis for pay negotiations, dropped from 5% in July to 4.8% in August.
The alternative consumer price index increased from 4.4% to 4.7%.
Mr Prentis said: "Although the RPI has fallen, there are still dark clouds on the horizon. The continued rise in the CPI underlines the real rising cost of basics like food and energy.
"Millions of public sector workers face a 2% pay cap and will be forced to make tough choices between heating or eating this winter.
"It is not too late for the government to levy a windfall tax on the energy and gas companies who are posting huge profits, paying fat dividends and putting up prices."