As soon as you discover it is missing you must report your lost or stolen passport to the local police and to the British Foreign and Commonwealth Office (FCO), Embassy or High Commission of the country you are in.
For details of the nearest FCO call +44 20 7008 1500 or visit the FCO website.
After reporting the loss or theft to the FCO office, you will need to complete a lost or stolen declaration form LS01 available from the FCO website . You can also obtain the form from the local FCO office. If your passport has been stolen make sure to get a police report as you will need the crime reference details for the completion of the LS01 form.
When you have completed the relevant paperwork, the FCO will record the loss of your passport and it will be cancelled to prevent someone else from using it. A new passport will then be issued to you so that you can continue your travels.