Britain is one of two EU states to hold one of the five permanent, veto-wielding seats on the UN Security Council.
Ambassador Matthew Rycroft said Britain would continue to align itself with shared EU positions on issues before the Security Council while Brexit talks are continuing.
And he promised that, even after London and Brussels have completed their divorce, Britain would retain its powerful permanent seat on the UN council.
"The United Kingdom continues to be a full member of the European Union until the end of the two-year process that the prime minister triggered today," Rycroft said.
"Whenever there are positions of the European Union we will faithfully represent them on the Security Council," he told reporters, after chairing a meeting of the body.
"We were a permanent member of the Security Council before we joined the EU. We were a permanent member as a member of the EU. And we will continue to be a permanent member of the Security Council after we leave the EU."
Asked whether British positions might diverge from those of its partners after Brexit is complete, Rycroft said: "That will depend on how the negotiation goes."
But he added: "I suspect you will not see a sudden change in the British interest, you will not be seeing a sudden change of British values."
Along with France, Britain is one of two EU states to hold one of the five permanent, veto-wielding seats on the UN Security Council.
Opponents of the Brexit decision have argued that it will dilute Britain's influence, at a time when powers like India are seeking a seat on the UN council.