Migrants are scrambling to secure British citizenship in record numbers amid fears Labour's looming immigration crackdown could make it far harder to settle permanently in the UK.More than 312,000 refugees, foreign workers and family dependants applied to become British citizens in the year to March - the highest figure ever recorded and nearly double the level seen eight years ago, according to new Home Office data.At the same time, a further 331,000 people sought indefinite leave to remain (ILR) - the route that allows migrants to stay permanently before applying for citizenship - over the past two years, marking another record high and a 28 per cent jump on the previous two-year period.The surge comes as Labour prepares to overhaul the immigration system by extending the amount of time migrants must spend in Britain before they can settle permanently.Under proposals being pushed by Shabana Mahmood, migrants would have to wait 10 years rather than five before qualifying for ILR unless they are high earners or employed in public services. Current plans would also apply the tougher rules to workers already living in Britain.Experts believe the prospect of stricter rules is fuelling a rush among migrants to secure citizenship while the existing system remains in place.Dr Nuni Jorgensen, of Oxford University's Migration Observatory, said the spike had exceeded expectations and was being seen not only among non-EU nationals but also among Americans and Europeans already living in Britain.