The IRS has declared that references to “husband and wife” in the US tax code can apply to married same-sex couples.

The Internal Revenue Service, which is responsible for federal taxation, published updated guidance this week.

The guidance reflects the outcome of the 2015 Supreme Court case Obergefell v Hodges, which brought equal marriage to all 50 states.

Under the new IRS guidance, references to spousal relationships in the tax code which refer to ‘husband and wife’ will be interpreted as referring to any married couple, including same-sex couples.

Regulation Section 301.7701-18 clarifies: “For federal tax purposes, the terms spouse, husband, and wife mean an individual lawfully married to another individual. The term husband and wife means two individuals lawfully married to each other.

“A marriage of two individuals is recognised for federal tax purposes if the marriage is recognised by the state, possession, or territory of the United States in which the marriage is entered into, regardless of domicile.”

Same-sex marriages were already treated equally for taxation purposes – the new guidance simply clarifies the code.

The guidance also clarified the position of civil unions and registered partnerships, which were introduced by some states before same-sex marriage, are not a form of marriage.

It says: “The terms spouse, husband, and wife do not include individuals who have entered into a registered domestic partnership, civil union, or other similar formal relationship not denominated as a marriage under the law of the state, possession, or territory of the United States where such relationship was entered into, regardless of domicile.

“The term husband and wife does not include couples who have entered into such a formal relationship, and the term marriage does not include such formal relationships.”