At Budget 2016 the government announced

At Budget 2016 the government announced that it will restrict the amount of profit that can be offset by carried-forward losses to 50% from 1 April 2017, whilst allowing greater flexibility over the types of profit that can be relieved by losses incurred after that date. The government ran a consultation on the reforms over the summer and, in responding to this, will take steps to address unintended consequences and simplify administration of the new rules.

More details, please following the link below:

https://www.gov.uk/government/consultations/reforms-to-corporation-tax-loss-relief-consultation-on-delivery