The three main political party leaders have thrown their support behind the United Nations' £4 billion appeal to help the millions of people forced to flee the Syrian civil war.
In a rare joint statement, David Cameron, Ed Miliband and Nick Clegg said it was essential that aid reached the refugees as the fate of an entire generation of Syrian children "hangs in the balance".
They pledged that Britain would contribute more on top of the £523 million already committed and urged other nations to follow suit.
"More than 9 million people in Syria are suffering this Christmas, in desperate need of our help. Four million of them are children - the equivalent of every primary school pupil in England. This is an urgent humanitarian issue that transcends the differences of party politics," they said.
"Our country has a long history of helping those in dire need. As a nation, we have already given £523 million - including many generous donations from members of the public. But even more is needed to cope with the scale of the suffering.
"The UN has recently launched a new call for £4 billion - the largest appeal in its history. Britain will give more - and we urge other nations to do the same."
At the same time, they said it was vital that all sides in the conflict guaranteed humanitarian access to ensure that help reached all those in need.
"Too many children are out of the reach of agencies like Unicef and the health authorities. Britain helped broker the agreement at the UN Security Council for unhindered access. Now it is essential to ensure this is delivered - with secure routes to get the life-saving aid to all those who need it," they said.
"The fate of an entire generation of children hangs in the balance. We must all do everything we can to help them. There is no better New Year's resolution."
The statement by the three leaders was welcomed by David Bull, the executive director of Unicef UK.
"Without the right supplies - clean water and warm blankets and clothes, vaccines and medicines - cold conditions could be fatal, especially for babies and young children," he said.
"Children who have suffered and witnessed so much also need psychological and emotional support and education.
"Unicef is increasingly concerned for children who are cut off from vital aid because of the ongoing conflict. Unicef will not rest until every child in need is reached."