Mr Obama has not committed to attending the talks next month
US President Barack Obama and other world leaders "need to" go to the environment summit in Copenhagen, cabinet minister Ed Miliband has said.
The climate change secretary told the BBC this was necessary to achieve the "kind of deal we want".
Mr Obama has said he will attend next month's UN-sponsored summit only if he is confident this will help countries reach a meaningful agreement.
UK Prime Minister Gordon Brown has promised to take part.
Mr Miliband acknowledged a legally binding deal was unlikely to be finalised at the two-week Copenhagen gathering, which runs from 7 to 18 December.
But he told BBC One's Andrew Marr Show: "It's a bit like when you buy a house. Exchange may happen at Copenhagen and completion some months afterwards.
Despite it being tough I am actually quite optimistic, because I think we have come a long way in a year
"What is most important, as far as I am concerned, is to get a really ambitious set of commitments from all world leaders."
Mr Brown has committed to going to Copenhagen for the final two days of the conference, after Danish prime minister Lars Lokke Rasmussen issued a formal invitation to 191 heads of state and government last week.
Mr Miliband said: "I think as many leaders as possible - including President Obama - do need to come there, because that will make a difference in the end to the kind of deal we want."
On Mr Obama, he said: "His diary is a matter for him, but I hope he does go. I think it's important that this is done in the end by leaders."
He also said: "If we can get a very clear set of commitments from the world leaders in Copenhagen on how they are going to cut their emissions - not just Europe, not just the US, but India, China and other countries - then that will be a very major step forward.
"So despite it being tough I am actually quite optimistic, because I think we have come a long way in a year.
"A year ago, many countries were saying this isn't really our priority concern - now we have China saying they want to be part of this, India saying it is going to take action, Brazil saying it will take action in relation to climate change."
Miliband: Copenhagen deal 'may not happen'
He added: "We want to get as far as we can at Copenhagen. The substantive commitments that leaders make are what matters. If we have to lock the lawyers in a room for a few months afterwards to turn that into a legal agreement, that's OK."
The Copenhagen talks are being held to build on the 1997 Kyoto Protocol's targets for reducing emissions, which currently apply only to a small set of countries and expire in 2012.
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