Barack Obama uses executive powers to force through immigration reform, removing the threat of deportation for roughly four million people. Photo: BBC

They include immigrants who have been in the US for five years and have children staying legally in the US.

About four million people are expected to benefit from a reform package forced through using executive orders, which allow Mr Obama to bypass Congress.

Republicans have accused the president of an "illegal power-grab".

There are estimated to be 11 million illegal immigrants in the US.

Under Mr Obama's plan, undocumented parents of children who are US citizens or legal residents will be able to apply for work permits lasting three years.

Only parents who have lived in the US for five years will qualify.

Some 3.7 million people are thought to be eligible for this scheme,according to the Migration Policy Institute.

Another part of the package will extend a programme that gives temporary legal status to people who arrived in the US as children.

Currently only those under the age of 30 who arrived before 2007 can apply for the programme, which was launched in 2012 and already covers roughly 1.2 million people.

Mr Obama has abolished the age limit and extended the cut-off point to 2010, potentially extending the programme to a further 300,000 people.

Analysis: Jon Sopel, BBC North America Editor

How has the immigration debate in the United States become so polarised, so toxic, so unpleasant?

That it has cannot be doubted. Barack Obama says he has grown so weary of trying to get Congress to engage seriously that he is going it alone.

He's bypassing the legislature and brandishing his big stick - the executive order. It is governance by diktat. And the reaction of his opponents suggests it will spark an atmosphere of retaliation and revenge.

How did immigration debate get so toxic?

The order will primarily affect immigrants with children who are US citizens, like Alejandra Mota (centre) and her son

Mr Obama said in a televised address that his measures would allow illegal immigrants to "come out of the shadows and get right with the law".

He insisted that his proposals, which are the biggest immigration reforms since the mid-1980s, did not amount to an amnesty.

"What I'm describing is accountability - a common-sense, middle ground approach," he said.

Republicans reacted with fury, with Arizona Republican Senator John McCain calling it an "illegal power-grab" that "fails to address the root causes of the dysfunction in our immigration system".

And Texas Governor Rick Perry argued the move would "lead to more illegal immigration, not less".

An Obama aide rebuffed the criticism that Mr Obama had overstepped his authority, saying the president had taken advice from the secretary of homeland security and the attorney general about the action.

"It's entirely consistent with the way previous presidents have exercised their executive authority," the aide said.

US illegal immigrants' country of origin (2012)

• Mexico - 59%
• El Salvador - 6%
• Guatemala - 5%
• Honduras - 3%
• Philippines - 3%

Source: Department of Homeland Security

Mr Obama's plan does not go as far as an immigration bill that failed to pass Congress last year.

The bill was supported by the Democrat-led Senate, but the Republican-controlled House of Representatives refused to debate the proposal.

It would have provided a path to citizenship for millions of undocumented immigrants.

Mr Obama stressed that his plans would not offer citizenship, or entitle immigrants to the same benefits as Americans.

Campaigners for migrants' rights broadly welcomed his plans.

But some activists worried that the promise of a three-year work visa would not be enough for many people to come out into the open.

"It's a step in the right direction, but it's going to fall far short of the mark," veteran advocate Cheryl Little told Reuters news agency.

She said the reforms amounted to "simply a temporary reprieve from deportation".

What is an executive order?

• Directive issued by the president to federal employees usually concerning the implementation of laws
• Power not explicitly outlined in the constitution, but exercised by most presidents
• Orders generally cannot force through major policy initiatives, which usually require an Act of Congress
• Courts can strike down an order if it is deemed incompatible with other laws or the constitution
• Up to October, President Obama had issued 193 orders in almost six years, a lower rate than most of his recent predecessors