Britain's opposition Labour party would move to nationalize key industries as a priority if it took office, its finance spokesman said ahead of its annual conference.

John McDonnell said Labour would set up Public Ownership Unit in the finance ministry "immediately" on entering government, and that in some cases investors might not be compensated.

"We're going to bring back into public ownership rail, water, energy and mail. We will get this legislation through in the first (policy program) Queen's Speech - we want to hit the deck running," he told the Daily Mirror ahead of the conference taking place in Liverpool this weekend.

Under socialist leader Jeremy Corbyn, a parliamentary veteran, and his would-be finance minister McDonnell, Labour has shifted from the centrist pro-business platform of former prime minister Tony Blair to a more interventionist left-wing pitch.

Nationalisation has long been a central tenet of their plans, which have previously been criticised by business.

McDonnell also told the BBC that while investors would usually be compensated, for some Private Finance Initiative (PFIs) deals that might not be the case.

"There might be some factors in relation to some operations of PFIs in particular that could be brought forward that says actually there is no compensatory arrangements needed here," he said.

The Conservative governments of the 1980s and 1990s privatised much of Britain's infrastructure, including the railway in 1997.

The current Conservative government has launched a "root and branch" review of the railway network after months of disruption to passengers caused by strikes, timetable changes and problems with the franchising system that was introduced after privatisation.

Finance minister Philip Hammond criticized McDonnell's plans. "Ideological renationalization would lead to less choice and poorer services," he said in a statement.