The marriage tax allowance which will take effect from 2015, will allow 420,000 Londoners, both gay and straight, to claim a transferrable tax allowance.
The law includes civil partners, and will include couples both gay and straight, and around 15,000 civil partners in the UK.
The Treasury unveiled information regarding the plan, which was announced by Prime Minister David Cameron earlier this year. Mr Cameron said it would benefit 4 million couples across the UK.
Tory MP Mike Freer said: “The values of marriage are give and take, support and sacrifice – they are values that we need more of in this country.
“That’s why Conservatives are backing marriage. Our £1000 marriage tax allowance will help 420,000 hard-pressed Londoners to keep more of their own money.
“Conservatives are on the side of every married couple: straight and gay, as well as civil partners. Love is love, commitment is commitment.”
The allowance could see couples up to £200 better off a year, and the benefits will be seen at the end of the 2016 tax year.