WASHINGTON (AFP) - US President Barack Obama will offer five million undocumented migrants protection from deportation, allowing families to come out of the shadows and seek work permits.

In a move guaranteed to infuriate Mr Obama's Republican critics, the White House said nearly all undocumented people living in country for more than five years and who have a child who is a US citizen or legal permanent resident can apply for three-year work authorisation.

The President will also broaden the programme he launched in 2012 that provides temporary residency to young undocumented immigrants who arrived in the United States before the age of 16.

"The President has gone as far as is legally allowable," a senior administration official told reporters, adding that those living and working illegally in the country and who meet the criteria can apply for deferred deportation from next spring.

Mr Obama's executive order shifts from a dragnet approach to re-prioritising the deportation focus on convicted felons and those who pose a danger to society. It will affect about 44 per cent of the 11.3 million people - mostly from Mexico and Central America - living in the shadows.

Since 1986, when then Republican president Ronald Reagan granted a sweeping amnesty, all attempts at major reform of the country's immigration system have failed.

Faced with a congressional stalemate, Mr Obama - who made immigration one of his priorities on taking office in 2009 - has decided, with two years left in the White House, to take the matter into his own hands.

The President will explain his plan to the nation in a prime-time speech at 8pm (0100 GMT Friday). He elaborated on his proposal in a speech at a Las Vegas high school.

"Everyone agrees that our immigration system is broken," he said in a video message on Wednesday. "Unfortunately, Washington has allowed the problem to fester for too long."

Under the new rules, those applying for deferred action must have a clean criminal record, pass a background check, and pay taxes, the White House said.

The plan expands the program allowing temporary residency cards for minors to include those of all ages, provided they arrived in country prior to Jan 1, 2010, instead of the previous date of 2007, and were 16 or younger when they entered.

All in all, the move would protect up to five million people from the threat of arrest and deportation by US federal authorities.

It may not, however, provide a clear path to eventual citizenship or permanent residency for migrants, an idea that may have to wait for another president or another generation of lawmakers.

The White House has long sought to pass a broad immigration reform package that would offer a path to citizenship for young migrants that grew up in the United States.

A new immigration law did pass the then-Democratically controlled Senate last year, but the Republican House of Representatives blocked it and failed to agree on its own alternative proposal.

White House lawyers and many outside experts believe Obama has the constitutional authority to act.

While other presidents have used executive powers to grant amnesty to undocumented migrants, none has ever taken such sweeping action.

Republicans, who will control both the House and Senate in January after a huge win in this month's midterm elections, say Obama is going too far.

And incoming Republican Senate Majority Leader Mitch McConnell warned Thursday that the new Congress will exact political retribution.

"If President Obama acts in defiance of the people and imposes his will on the country, Congress will act," he said.

"We're considering a variety of options. But make no mistake. When the newly elected representatives of the people take their seats, they will act."

But Democrats counter that those same Republicans have blocked other attempts at reform.

According to an opinion poll conducted by NBC and The Wall Street Journal, 48 per cent of Americans disapprove of Obama's immigration plans, against 38 per cent who back them.

The political firestorm unleashed by Obama does not bode well for relations between Congress and the White House in coming months.

Republicans cannot block a presidential decree, but they can make Mr Obama's last two years extremely difficult - by blocking his choices to fill ambassadorial and administration posts, as well as judgeships.

But with the 2016 presidential election on the horizon, the debate within the party on immigration will be lively, as Republicans can ill afford to ignore Hispanic voters, 70 per cent of whom voted for Obama in 2012.