The New EU Changes
David Cameron fought hard to get a four year ban on new arrivals from other EU nations claiming in-work benefits, such as tax credits, which top up low wages.
It will not be a blanket ban, as he had wanted, but will be graduated, so the longer migrants work in the UK, the more benefits they will become entitled to.
The prime minister insists this will "make a difference" to high levels of immigration by reducing one of the main "pull factors" attracting people to the UK.
But there is little direct evidence to suggest it will work.
Sir Stephen Nickell, a senior economist at the Office for Budget Responsibility, said last year he believed it was "unlikely to have a huge impact".
The main "pull factor" for migrants is the availability of jobs, according to Oxford University's Migration Observatory, and campaign group Migration Watch.
Benefits might prove to be an attraction to some, but most EU-born migrant workers are single or childless couples who do not claim a lot of tax credits, researchers say.
Migration Watch said Mr Cameron's "emergency brake" on benefits would be "unlikely to have any significant effect on levels of migration".
Some campaigners have suggested that increasing the amount of benefits people can receive each year will encourage them to remain in Britain.
The in-work benefit ban was dreamed up by influential think tank Open Europe (its former boss Mats Persson is now Mr Cameron's chief adviser on EU reform).
It argued in a November 2014 paper, that banning in-work benefits would mean take-home pay could drop below that which migrants would receive in their home states - they used Poland, Bulgaria and Spain as their examples - meaning that a move to the UK would no longer make financial sense.
But the UK's plan to increase its minimum wage to £9 an hour for over-25s by 2020, which the government claims will remove the need to top-up wages with benefits, could undermine that argument.
Also, it is not clear how checks would be made on how long people have actually been in the UK. Some regularly travel in and out of the UK or return to their home country to see family for a few months at a time.
Asked whether Mr Cameron believed the proposed 'emergency brake' will cut immigration, a Downing Street spokesman said that migrant families were currently able to claim an average of £6,000 a year in tax credits and some 10,000 were claiming £10,000 or more.