Angelina Jolie is "a pleasure to work with", Foreign Secretary William Hague has said.
Mr Hague said the Hollywood star is able to reach people that governments can not, and praised her for her "passion".
He and Angelina are co-chairing the global summit to End Sexual Violence in Conflict (ESVC), and have launched an international protocol which they hope will "really, truly end impunity".
Speaking about the Maleficent actress, who is special envoy for the UN High Commissioner for Refugees, Mr Hague said: "She's great to work with because she is very professional, very knowledgeable, full of ideas.
"We're used to working together now of course because we've made a number of international visits together and co-founded the preventing sexual violence initiative.
"And she brings what governments can't. I've always said you need a big government of the world to be involved in this, which was a missing ingredient in all the previous work by the UN and NGOs on sexual violence."
He added: "But you need something much more than government, you need to be able to reach people who don't easily listen to governments, and Angelina really brings that, as well as bringing great knowledge and passion about these issues.
"So I find her a pleasure to work with."
The protocol - primarily a document outlining basic standards of best practice - offers guidance for those on the frontline who document sexual violence as an international crime.
It sets out how to recognise sexual violence as an international crime and how to investigate and document it. It also sets out how to protect victims, witnesses, interviews and information on sexual violence.
At the launch Angelina said she has met survivors of war zone rape all around the world and they all ask for one thing - "Justice".
The star said war zone rape is a "crime against humanity" and said the rights of survivors - to be accepted by society, to be supported, and to see their attackers held accountable before the law - are denied to millions of people around the world.
"The number of convictions for warzone sexual rape is pitifully small," she said.
Angelina said at the heart of her and Mr Hague's campaign they are calling for an end to impunity.
"Perpetrators have to know that even during conflict, evidence is being collected and it will be used against them. They have to know that when peace agreements are made there will be no amnesty for rape," she said.
The actress added: "We all know how hard it is to secure convictions for rape even in stable democratic countries so we have to work even harder to make justice possible in fragile countries. And that is the purpose of this protocol. It is an essential document."
She said she is "so grateful" to the hundreds of experts and survivors who played their part in drafting the protocol.
Angelina said she and Mr Hague are "determined" to work to ensure that it is implemented and said they will be asking all the governments coming to the summit tomorrow to adopt the protocol.
The actress ended her speech by saying: "I hope that by all of us working together we can support survivors for the long, but not impossible, road to justice. And finally, really, truly end impunity."