Actress Carey Mulligan has urged Theresa May to formulate a "clear plan" to put an end to the "brutal" ongoing conflict in Syria.
The Oscar-nominated star appears in a new video campaign for War Child along with a host of other celebrities calling for an end to the suffering of children in areas that have been affected by the war as the conflict reaches its seventh year.
Mulligan, the charity's Global Ambassador, said the British government needs to outline what the UK is going to do to help people, particularly children, who have had their lives devastated by the war.
She said that the UK needs to show that it will "put pressure on the Syrian regime and other powers" in the fight to stop the war - which War Child says has so far killed up to 55,000 children, the equivalent of one child for each hour the war has taken place - since it began on March 15 2011.
Mulligan told the Press Association: "The Prime Minister needs to come to Parliament with a clear plan to help bring this brutal conflict to an end.
"This should be clear about what the UK will do to help stop attacks on children, and hold those to account who are guilty of these crimes.
"It should demonstrate how we will ensure that humanitarian aid gets to those who need it, and work with other countries to ensure there is enough funding for the scale of the crisis."
She added: "Importantly, the UK must outline what it will do to put pressure on the Syrian regime and other powers to stop the fighting and reach a political solution."
Mulligan said that those who want to help should write to their local MP to "keep the conflict in the front of mind".
Mulligan appears in War Child's Enough is Enough video along with actor Jude Law and singer Nicole Scherzinger among other stars, and sees them read out the harrowing account of 17-year-old Fatima, a Syrian girl whose life was turned upside down by the war.
Fatima was forced to flee her home along with her family when conflict broke out in Syria, and is now living as a refugee in neighbouring Jordan.
In the video - which also stars comedian Michael McIntyre and musicians Sam Smith, Marcus Mumford and James Bay - Scherzinger reads part of Fatima's account.
She says: "Syria was being destroyed and we were fleeing because we wanted to save our lives."
Mulligan picks up the tale: "Once we came to Jordan life was very depressing, we were like flowers or roses pulled from their place and thrown into the desert."
Fatima later learned that one of her brothers had been killed, and Law says: "Tears started coming down my face, they burned me."
To coincide with the video release, War Child has released a new report entitled Six Years On - A War On Syria's Children, which says that over 400,000 people have been killed since the conflict began.
The report says that 24,578 deaths of children have been documented but that it is likely there are many more than this, as many as 55,000 as an upper estimate, from bombings as well as other war-related causes.
More children have been killed in Syria than British people during the Blitz and by March 16, the war will have lasted as long as the Second World War.
:: War Child's Enough is Enough video can be viewed at www.goo.gl/3igcl0