People are being urged to join UK landmarks as they switch off their lights for an hour on Saturday night to back action on climate change.
Talks With PM On More Powers
David Cameron will tell the leaders of Scotland, Wales and Northern Ireland that it is time to concentrate on best using the powers they have rather than debating what more could be devolved.
The Prime Minister hopes to focus a session of the Joint Ministerial Committee on UK-wide issues - including a briefing from security and intelligence services about the threat posed by Islamic State-inspired terrorism.
"It is now time we reached a point where the big debate is about how the powers are used, not about what they should be,'' he said ahead of the annual meeting at Downing Street.
The Downing Street gathering is the first face-to-face meeting between the Prime Minister and Nicola Sturgeon since she became Scotland's First Minister and will be followed by direct talks between the two.
It will also put Mr Cameron back across the table with Northern Ireland's First and Deputy First Ministers Peter Robinson and Martin McGuinness for the first time since his direct intervention in Belfast last week failed to secure any breakthrough in talks aimed at resolving a series of major disputes.
Welsh First Minister Carwyn Jones will also be present for a briefing from the Joint Terrorism Analysis Centre (JTAC), which in the summer raised the official threat risk to the UK from substantial to severe.
Number 10 said the aim was to foster a better understanding of the challenges posed and closer cooperation on tackling extremism at home and abroad.
It is the first time the leaders have come together since Scotland vetoed independence and Edinburgh was promised significant new powers, legislation was passed extending Cardiff's tax and borrowing powers and Belfast was told progress in talks was the key to being handed control of corporation tax.
The Government is also expected to publish proposals this week for dealing with the English Votes for English Laws issue.
"We have shown that this Government is determined to address the complex constitutional issues with settlements that are fair to all and built to last,'' Mr Cameron said.
"We are keeping our promises, delivering on our commitments and making the United Kingdom all the stronger for it.
"Now is a chance to move forward, forging even stronger ties between our governments and our parliaments, and working together on the big issues across all the nations from the economy and trade, to global issues like aid and Ebola.''
In their one-to-one talks, Ms Sturgeon will discuss the Smith Commission agreement, including the proposal that the Scottish Parliament be given powers over its own elections, which could pave the way for 16 and 17-year-olds to vote in Holyrood elections.
Speaking ahead of the talks, Ms Sturgeon said: "Giving 16 and 17-year-olds the vote for the independence referendum is widely seen as having been a huge success, which added to the unprecedented democratic engagement of the campaign and the massive turnout.
"We want to make sure those same young adults now have the chance to vote in the next Scottish Parliament election, and have their say on how the country should be run.
"But to do that we need agreement from Westminster - and that's why tomorrow's talks with the Prime Minister are so important.
"I will make it clear to David Cameron that we need to see swift action to make sure the powers are transferred in time to allow 16 and 17-year-olds to have a vote in the next Holyrood election.
"Any delay by Westminster in taking forward the necessary steps could see them deprived of a vote - but I am very confident that will not happen and that myself and the Prime Minister will reach a deal.''
She said the issue was a "key early test'' of Westminster's commitment to delivering the additional powers proposed by the Smith Commission.
"While those powers are not nearly as extensive as we would have wished, we do welcome them and now need to see them delivered as quickly as possible,'' she added.
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