asked Her Majesty's Government:

Whether a person gaining citizenship at a citizenship ceremony can also be a citizen of another sovereign state; and, if so, to which state that person is required to give his first loyalty. [HL1177]

A foreign national is not required under UK law to relinquish his original nationality on becoming a British citizen. However, prospective British citizens are required to pledge allegiance to Her Majesty the Queen, to give their loyalty to the United Kingdom and to undertake to respect its rights and freedoms.