Scroll down to 319E at
http://www.ukba.homeoffice.gov.uk/polic ... 1migrants/
It can't get any more 'OFFICIAL' than this.
Pay particular attention to :
319E (b) The applicant must be the spouse or civil partner, unmarried or same-sex partner of a person who:
(i) has indefinite leave to remain as a Relevant Points Based System Migrant;
319E (c) The applicant must have, or have last been granted, leave as the partner of the Relevant Points Based System Migrant who:
(i) has indefinite leave to remain as a Relevant Points Based System Migrant;
319E (d) The applicant and the Relevant Points Based System Migrant must have been living together in the UK in a marriage or civil partnership, or in a relationship similar to marriage or civil partnership, for at least the period specified in (i) or (ii):
(i) If the applicant was granted leave as:
(a) the Partner of that Relevant Points Based System Migrant, or
(b) the spouse or civil partner, unmarried or same-sex partner of that person at a time when that person had leave under another category of these Rules
under the Rules in place before 9 July 2012, and since then has had continuous leave as the Partner of that Relevant Points based System Migrant, the specified period is 2 years .