Many Muslims in the UK apply for British nationality and passports and this involves signing an oath (Kasam). I do not have the details of the oath but I understand that it includes promises to be sincere to the Queen and Britain. Is this Halaal, Makrooh or Haraam?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

There is no doubt that a person who accepts the naturalization of disbelieving countries commits many religious infractions.

Among these infractions is to utter what is not permissible to believe in or abide by, like accepting their regime which is totally different from Islam, and uttering an oath to be loyal and friendly with them. This will also oblige him to serve in their army if military service is obligatory in their country, as well as fight on their side against their enemies even if these enemies are his brothers in religion and creed, not to mention many more obligations which contradict Islam.

Therefore, applying for naturalization should be only for a dire necessity or a considerable religious benefit.  The Muslim has also to avoid, as much as possible, the forbidden matters that result from naturalization, and he must be able to openly practice the rites of his religion. 

Allaah knows best.