"We're suing President Trump to stop him from unilaterally robbing taxpayer funds lawfully set aside by Congress for the people of our states. For most of us, the office of the presidency is not a place for theatre," California Attorney General Xavier Becerra said.

Joining California in the lawsuit were Colorado, Connecticut, Delaware, Hawaii, Illinois, Maine, Maryland, Minnesota, Nevada, New Jersey, New Mexico, New York, Oregon, Virginia and Michigan.

The states - all of them but Maryland governed by Democrats - say the lawsuit aims to protect their residents, natural resources and economic interests. They argue that Mr Trump's order to divert funds would cost them millions of dollars.

The White House has not commented on the lawsuit, filed in the court for the Northern District of California.

California Governor Gavin Newsom dismissed the president's decision as "political theatre" while New York state's Democratic Attorney General Letitia James promised to "fight back with every legal tool at our disposal".

The president said the emergency would allow him to get almost $8bn for the wall, still considerably short of the estimated $23bn cost of the barrier along almost 2,000 miles (3,200km) of border.

Mr Trump accepted that he would be sued for the move, and predicted that the case was likely to end up in the Supreme Court.

"We're going to confront the national security crisis on our southern border," said Mr Trump, who repeatedly declared during the 2016 presidential campaign that Mexico would pay for the wall. "Everyone knows that walls work."

Critics, however, have accused the government of manufacturing a crisis, saying the president's assertions about the situation are untrue.

Experts say the largest number of illegal migrants settling in the US each year is those who stay in the country after their visas expire, and that most drugs are trafficked into the US through ports of entry.

Following the declaration on Friday, a liberal advocacy group, Public Citizen, sued on behalf of a nature reserve and three Texas landowners who were told the wall could be constructed on their properties.

The National Emergencies Act is intended for times of national crisis. Declaring it gives the president access to special powers that effectively allow him to bypass the usual political process.

Mr Trump could be able to divert money from existing military or disaster relief budgets to pay for the wall.

Emergency declarations by previous presidents have been overwhelmingly used for addressing foreign policy crises - including blocking terrorism-linked entities from accessing funds or prohibiting investment in nations associated with human rights abuses.

The emergencies act contains a clause that allows Congress to terminate the emergency status if both houses vote for it - and the president does not veto.

With a comfortable majority in the House, Democrats could pass such a resolution to the Senate. The Republicans control the Senate, but a number of Republican senators have been vocal in their unease about the president invoking a national emergency in this case.

The resolution would however still require Mr Trump's signature to pass, allowing him to veto it. A supermajority in both houses of Congress - currently in a weeklong recess - is needed to overturn a presidential veto.