European Council President Donald Tusk presented his "new settlement" proposal in talks with Britain on reforms to help keep the country in the 28-nation bloc.

Responding to Prime Minister David Cameron's four objectives put to him in November, Tusk described keeping the unity of the EU as the "biggest challenge".

"The line I did not cross, however, were the principles on which the European project is founded," he added.

Here is the letter in full: