Britions in South Sudan are being urged to flee the country as the UK charters its final evacuation flight to the troubled African state.
The Foreign Office said a third plane was being sent to the capital city of Juba tomorrow and warned that the government would struggle to help anyone who chose to stay behind.
Fighting has spread through the newly-formed East African state, which gained its independence in 2011, following a reported coup attempt in the capital last weekend.
British military transport planes have been used to evacuate two groups of UK nationals over recent days and a United States rescue plane was hit by incoming fire leaving three American service personnel wounded.
The United Nations has estimated that up to 500 people have been killed in fighting between rival factions following a coup attempt against the president by soldiers loyal to his former deputy.
A Foreign Office spokesman said: "Due to the continued violence in South Sudan, the Foreign and Commonwealth Office (FCO) is planning to send a third and final plane to assist any remaining British nationals to leave.
"The FCO is planning to provide a charter flight to depart from Juba to Dubai during the afternoon of Monday 23 December. Following the assisted departure of British nationals earlier this week on military flights, this will be the final flight provided by the FCO for British nationals to leave South Sudan.
"If you are a British national and you wish to leave Juba you should contact the FCO as soon as possible on +44 207 008 1500 or by email at firstname.lastname@example.org.
"If you would like to be on the flight on 23 December, you will need to be at Juba Airport by 1200 hours. It may not be possible to carry baggage. If you bring baggage to the airport, you may have to leave it there.
"We strongly advise all British nationals in South Sudan to leave the country if they can do so safely. You may have difficulty leaving in the event of a further deterioration in security.
"British nationals choosing to remain in South Sudan should remain alert to the local security situation, monitor the media, and stay in a safe location. A curfew is in place in Juba and some other towns between 6pm and 6am. You should be aware that it will be difficult for the British government to provide consular assistance in the event of a further deterioration in the security situation.
"Any British nationals wishing to leave should be reassured that all personal information will be protected in line with the Data Protection Act 1998."
The Archbishop of Canterbury, the Most Rev Justin Welby, and the Vatican called for the nation's weak and poor to be "spared the trauma of conflict".
In a joint statement they said: "On behalf of our churches we appeal to the leaders in South Sudan to lay down their weapons and co-operate in seeking peace through dialogue and negotiation. As we approach the celebration of the birth of our Saviour, the Prince of Peace, this is a time for the vulnerable, the weak and the poor to be spared the trauma of civil conflict.
"We bear in mind the call of the Holy Father for peace in 2014, especially because in God's design at creation, we are all brothers, the sons and daughters of the one God and Father.
"Let us not yield to fratricidal tendencies in the name of our ethnic differences (which is what God created us to be). With all Christians, we pray for peace in South Sudan. We pray that all of you in South Sudan may become instruments of peace.
"We issue this appeal together, knowing that Christian leaders in South Sudan are working in unity and courage of faith for peace. "