OTTAWA -- Liberal MP Sean Fraser says it's premature for the Liberal government to say how a promised new clean-fuel standard will affect gasoline prices, as consultations on how the policy will be implemented are still underway.

Fraser, who is parliamentary secretary to the environment minister, held a press conference today with fellow Liberal MP Mona Fortier to counter Conservative rhetoric against the fuel standards and against the Trudeau government's environmental policies more generally.

He said the Liberal plan to bring in new clean-fuel standards -- which will force cleaner-burning fuels to lower carbon emissions by 30 million tonnes a year -- is not the "secret fuel tax" Conservative Leader Andrew Scheer has branded it.

But he would not say how much the new standards might affect prices at the pumps.

Fraser says regulations on the standards are still being discussed with provinces, territories and the public and this input could lead to "tweaks" to ensure prices remain affordable for all Canadians.

On Monday, Scheer sent a letter to Prime Minister Justin Trudeau saying he would scrap the Liberals' fuel standards if his party wins the fall federal election, surmising they could increase the cost of gas by at least four cents a litre over and above the government's carbon tax.