US President Donald Trump is to declare a national emergency to fund his planned border wall with Mexico, the White House has said.

He will sign a border security bill to avert a government shutdown - but also act to bypass Congress and use military funds for the wall, a statement said.

Senior Democrats have responded by accusing him of committing a "gross abuse of power" and a "lawless act".

Building the wall was a key election promise by Mr Trump.

But as president he has so far been unable to get the necessary funding to advance it.

Congress passed the bill - which does not meet Mr Trump's demands for wall funding - on Thursday. It now has to be signed by Mr Trump to become law.

• Reaction to Trump's decision to declare national emergency

The president insists the US needs a physical barrier on its southern border to stop the flow of migrants, which he says has reached crisis levels.

But critics say the number of migrants at the border is far lower than in previous decades, and that most are trying to enter the US legally.

What did the White House say?

"The president is once again delivering on his promise to build the wall, protect the border, and secure our great country," White House Press Secretary Sarah Sanders said in a statement on Thursday.

She added he would "take other executive action - including a national emergency - to ensure we stop the national security and humanitarian crisis at the border".

The compromise legislation passed by Congress includes $1.3bn (£1bn) in funding for border security, including physical barriers, but it does not allot money towards Mr Trump's wall. Mr Trump had wanted $5.7bn for this.

• Can Trump use emergency powers to build wall?
• Is there a crisis on the US-Mexico border?

When Mr Trump warned that he might declare a national emergency over his wall earlier this year, some Republicans argued it would set a dangerous precedent.

Speaking on the Senate floor on Thursday, however, Republican Majority Leader Mitch McConnell indicated his support for the move, saying the president was taking action with "whatever tools he can legally use to enhance his efforts to secure the border".

In a 83-16 vote, the Senate on Thursday passed the border security bill. The House of Representatives later also backed the measure, by 300 to 128.

How have Democrats responded?

Speaker of the House Nancy Pelosi has already suggested a legal challenge from Democrats should the president make an emergency declaration.

She and Senate Democratic leader Chuck Schumer also issued a strongly worded joint statement condemning the move.

"Declaring a national emergency would be a lawless act, a gross abuse of the power of the presidency and a desperate attempt to distract from the fact that President Trump broke his core promise to have Mexico pay for his wall," they said.

• Trump's border wall in seven charts
• How much of the wall is built already?

"He couldn't convince Mexico, the American people or their elected representatives to pay for his ineffective and expensive wall, so now he's trying an end-run around Congress in a desperate attempt to put taxpayers on the hook for it."

The Democrats vowed that Congress would "defend our constitutional authorities".

What is a national emergency?

A state of emergency is declared in times of crisis. In this case, Mr Trump says the crisis is being caused by migrants arriving on the US-Mexico border.

Experts say declaring a national emergency would give the president access to special powers that effectively allow him to bypass the usual political process.

He would be able to divert money from existing military or disaster relief budgets to pay for the wall.

However there is debate about whether the situation at the southern US border constitutes such an emergency.

On the one hand, more than 2,000 people were turned away or arrested at the border each day during November alone. Supporters say this equals an emergency.

Others argue the figure is far lower than a decade ago, and many of the thousands of people who travelled north from countries like Honduras are presenting themselves as asylum seekers, looking to enter the country legally.

BBC.COM