Chancellor Gordon Brown and Tory leader David Cameron are to set out rival visions for tackling climate change as they grapple for leadership of the "green" agenda.
The two men, who are expected to lead their parties into the next General Election, are both delivering keynote speeches on the environment - seen by party strategists on both sides as one of the defining electoral battlegrounds.
Mr Brown, who is speaking later in the day in London, will use his speech to warn that the Tories' continuing Euro-scepticism disqualifies them as an effective force when it comes to dealing with global warming.
He will argue that real progress can only be achieved by working through bodies like the European Union to reach international agreements on curbing damaging greenhouse gas emissions.
Mr Cameron, who is addressing a Tory Party environmental summit, will challenge the Government to set annual targets for cutting emissions when it publishes its Climate Change Bill on Tuesday - something ministers have so far resisted.
"We must make sure that the measures announced by the Government have real bite - that they are not just greenwash," he will say.
"Without annual rate of change targets, it is too easy for the timetable to slip. And once it has slipped, it is much harder to make up the difference later."
He will say that to be credible and effective, targets must be independently set and monitored.
"The long-term nature of the climate change challenge demands a framework and disciplines that no government, of any political colour, will ever be able to fudge," he will say.
"The Climate Change Bill needs teeth and we will do all we can to improve the Bill as it goes through Parliament."
