Members of Syria's Palestinian population have become the conflict's "double refugees", says the Disasters Emergency Committee.
Palestinians settled in Syria have been forced to flee their homes along with the rest of the population as a result of the ongoing conflict.
Around 33,000 Palestinians have sought refuge in neighbouring Lebanon where they face overcrowding and poor conditions in pre-existing camps for Palestinian refugees.
These camp were established by the United Nations Relief and Works Agency (UNRWA) for Palestinian refugees, who left Palestine after the 1948 Arab-Israeli war.
The UNRWA estimates at least a fifth of the houses in the camps are not fit to live in, with many lacking walls, roofs, windows or toilet facilities.
In camps where many homes do not have water or electricity, there are concerns that the increase in refugees will lead to water shortages and trigger hygiene issues.
Palestinians already living in the camps are doing what they can to support the new arrivals, but as Palestinians are not permitted to work in many professions in Lebanon, they mostly work in low-paid jobs, meaning their own resources are thin.
Oliver Pearce, Christian Aid's Middle East Programme Manager said: "Camps are overcrowded with as many as 15 people living in one room. Many are ill and malnourished. Inflation and the lack of jobs means refugees can't afford to eat properly, children are facing years out of education."
The UNRWA is the only UN agency mandated to support Palestinian refugees but its resources are stretched, says the DEC.
Christian Aid and other DEC member agencies are working to provide assistance to Palestinian refugees at risk of falling through the support net.
Mr Pearce said: "The Palestinian refugee camps in Lebanon are very stretched in terms of infrastructure and capacity to support new arrivals and UNRWA doesn't have sufficient funding to meet all their needs.
"Camps are overcrowded with as many as 15 people living in one room. Many are ill and malnourished. Inflation and the lack of jobs means refugees can't afford to eat properly, children are facing years out of education."
Christian Aid and Oxfam partner, Association Najdeh, has been providing mattresses, food, counselling and education for children to more than 3,500 Palestinian refugees.
"Palestinians who've recently fled their homes in Syria have become double refugees," said Francis Lacasse, Oxfam's Syria crisis response manager.
"They're highly vulnerable, and many are in danger of falling through the aid safety net and not getting the help they urgently need."
To donate to the DEC Syria Crisis Appeal visit www.dec.co.uk