A retrospective change to immigration rules could erode public trust in government, the Institute for Government has warned in a new analysis of a decision taken by Shabana Mahmood.

The think tank's intervention centres on a measure that applies immigration consequences with effect from the past rather than only from the date of announcement. Mahmood, a senior Treasury figure whose portfolio the analysis examines, backed action that reaches back over decisions migrants had already made.

The Institute for Government argues that this kind of retrospective rulemaking carries a cost beyond the individuals it touches. When the state changes the terms of past conduct, the analysis contends, people lose the ability to trust that today's rules will still apply tomorrow.

What does retrospective action change?

Under ordinary practice, immigration rules bind from the date they take effect. Migrants plan applications, deadlines and travel around the rules in force at the time.

Retrospective action breaks that assumption. It re-evaluates past conduct against new standards, which can alter the legal position of people who complied with the rules as they stood.

The Institute for Government's warning rests on a governance principle rather than a single case outcome: governments that rewrite the past invite scepticism about every future rule they publish.

Who is affected?

The analysis addresses the consequences for people whose immigration position the retrospective measure touches, as well as for the wider credibility of the Home Office and HM Treasury decision-making.

The think tank does not dispute that governments can legislate in this way. Its argument is that they should weigh the trust cost before they do.

Readers seeking authoritative guidance on how the measure applies to individual cases should consult the official government source for the change and qualified immigration advisers, as this analysis addresses policy consequences, not personal circumstances.

The full argument appears in the Institute for Government's published commentary on Shabana Mahmood's retrospective action on immigration.