In June, a 30-year-old Sri Lankan man was detained as he prepared to take part in a suspected sham civil partnership ceremony at Ealing Register Office.

BBC: UKBA alerted over suspected sham civil partnerships

"As the demand for same-sex marriage is uncertain, and because these economic benefits are not guaranteed to be additional (for example, the spending on a civil partnership or marriage ceremony may not be additional if the couple and guests cut back on spending in other areas), we use zero benefit as our best estimate, " it explains.

BBC: Gay marriage: Eight things for better, for worse