"I (...name...) swear by Almighty God that I will be faithful and bear true allegiance to Her Majesty Queen Elizabeth, her heirs and successors, according to law. So help me God." - certain variations are allowed (see here for more details).
This oath of allegiance does of course present problems to those who believe that Britain should be a republic, or who do not acknowledge allegiance to the United Kingdom. For this reason Sinn Fein MPs do not take their seats. A member who fails to take the oath cannot take their seat & participate in the work of the House of Commons.
Various devices have been used by MPs who believe in a Republic, but do not wish to disenfranchise their constitutents (and themselves), to "overcome" the words they object to. Some have whispered additional words, but this is frowned upon - and may be illegal. Others cross their fingers while taking the oath. This device became less effective when cameras were allowed in. The photograph with todays post shows Tony Banks using this device - and in 1997 it hit the front pages (he was a Minister).
A facinating House of Commons Research Paper on the history and rules relating to the oath can be read here. I hope to publish a post on Charles Bradlaugh, who was the MP for Northampton, in the near future.