Image copyright Getty Images Image caption Nearly two thirds of voters approved a ballot measure to legalise marijuana in Washington DC

Washington DC has become the latest place in the United States to legalise the possession of small amounts of marijuana.

As of midnight on Thursday (05:00 GMT), people who use the drug in private no longer face prosecution.

The change has created tension between the city's mayor and Congress.

Washington DC joins Alaska, Colorado, and Washington state as the only places in the US that allow the use of the drug for recreational purposes.

Residents and visitors to the city over the age of 21 can possess as much as 2oz (56g) of cannabis, and may grow a few plants at home.

Buying and selling the drug remains illegal, as does smoking it in public.

The plan was overwhelmingly agreed in a referendum last November.

Image copyright Getty Images Image caption Representative Chaffetz warned the city's mayor against legalising the drug

But the vote revived tensions between local officials and Congress.

Washington DC - a federal district, not a state - is required to seek congressional approval for much of its legislation.

In a letter sent on Tuesday, two members of Congress warned Mayor Muriel Bowser that she would be breaking US law by proceeding.

They said that a national budget bill passed in December prevents the legalisation of marijuana in Washington.

But Ms Bowser and other officials believe that the legalisation is still valid since it was approved by voters before Congress passed the budget bill.

Image copyright Getty Images Image caption Washington DC Mayor Muriel Bowser has defied a warning from members of Congress

In the letter, the congressmen warned her that by enacting the new rules she would "be doing so in knowing and wilful violation of the law".

Speaking to the Washington Post newspaper, Representative Jason Chaffetz, one of the letter's signatories, warned that she could face "very severe consequences", adding: "You can go to prison for this."

Ms Bowser said: "We do disagree on a matter of law. There are reasonable ways to resolve that without us threatening him or he us."

While any criminal prosecution would have to come from the US Department of Justice, Congress could withhold Washington DC's funding for other initiatives to pressure Ms Bowser.