Shabana Mahmood’s beleaguered immigration reforms are set to face a new setback as migrant campaigners prepare to launch a major new legal challenge.

The Home Secretary plans to make migrants wait longer for the right to live in Britain indefinitely.

But the changes, which have yet to be announced in detail, will face immediate legal action as soon as they are unveiled.

It comes amid growing opposition to the plans from the Left of the Labour party, with former deputy prime minister Angela Rayner last week describing them as ‘un-British’.

The legal challenge, which has already raised £25,000 from an online ‘crowdfunding’ campaign, will have the potential to slow down Ms Mahmood’s attempts to introduce reform.

Currently, migrants – such as foreign workers – must live here legally for five years before applying for settlement to stay here permanently, known as ‘indefinite leave to remain’ (ILR).

Plans unveiled in November said that would be extended to 10 years, and to 20 years for refugees.

Labour’s Left wing has been angered by the retrospective nature of the proposals which mean migrants already living here will have to wait years longer to win ILR and secure full access to welfare benefits and social housing.

Home Secretary Shabana Mahmood has yet to reveal full details of the changes to ‘indefinite leave to remain’, but the outlined proposals have angered the Left of her party

Labour backbenchers have even suggested the policy will lead to a rise in child poverty, even though migrant workers are free to leave at any time and return to their own countries.

Ms Rayner said last week that ‘moving the goalposts… undermines our sense of fair play’, sparking fresh claims she is positioning herself to replace Sir Keir as Prime Minister following May’s local elections, in which Labour are expected to perform poorly.

She added that the plans had led migrants to ‘fear for their future’.

Ms Mahmood has insisted the changes are necessary because two million migrants are due to become eligible for ILR from next year, in a multi-billion pound drain on public finances.

Officials have denied reports that Ms Mahmood has threatened to resign if her policy is watered down.

Now pressure group the Skill Migrants Alliance (SMA) is preparing to bring a legal challenge and has instructed solicitors and a leading barrister to work on the case.

It will focus on the retrospective aspect of the changes.

The work is being led by Sonali Naik, KC, the joint head of Garden Court Chambers, who has already been involved in several successful legal challenges against Home Office immigration policy.

The SMA’s legal team will submit a ‘pre-action protocol’ letter to the Home Office as soon as the Government formally publishes its proposals, expected later this year.

As SMA spokesman said: ‘Should the government decline to modify its retrospective approach, the group is equipped to pursue judicial review proceedings.

‘By securing the best legal counsel, we are sending a clear message to the Home Office: we are prepared, we are professional, and we are united.’

The Home Office was approached for comment.