The environmental plans of big carbon emitters are coming under increasing scrutiny from investors and climate change activists concerned about such companies' ability to meet carbon targets, with the UK government targeting net zero carbon emissions in Britain by 2050.

Go-Ahead, which runs buses in London and several UK regions as well as operating train services in southeast England, said it will cut emissions by 75% by 2035 and to net zero by 2045.

The group said it will buy electric and hydrogen fuel cell buses as it gradually replaces its fleet of 5,000 buses with zero-emission vehicles by 2035 while also making all of its trains electric by that date.

That will require a substantial investment on Go-Ahead's part, with electric buses costing roughly twice as much as diesel alternatives, though prices are expected to fall over time. Go-Ahead will also need to invest in charging points at bus depots.

Chief Executive David Brown said radical change is needed in the push for net-zero emissions.

"We hope our commitment will be matched by a broader shift in public policy on transport by encouraging people to walk or cycle where possible, use buses and trains as an alternative but only use a car if absolutely necessary," he said in a statement.

(Reporting by Sarah Young; Editing by David Goodman)