Britain's main Labour opposition on Thursday vowed to renationalise the UK's much-maligned rail services within five years if it wins a general election due later this year.

Private companies currently run many of the rail networks but Labour said it would fold them into a single, publicly owned body when their contracts expire.

The announcement marked a fairly rare concrete pledge from the party tipped to win the nationwide vote that is expected in the second half of the year.

Labour has been pretty light on policy detail as it tries not to blow a huge lead over Prime Minister Rishi Sunak's Conservatives in opinion polls, and return to power after 14 years in opposition.

"We will establish Great British Railways -- a single, directing mind to control our railways in the passenger interest," said Labour's transport spokeswoman Louise Haigh.

Britain's railways have been hit by a wave of strikes in recent years caused by cost-of-living pressures.

Train cancellations are commonplace and hard-pressed passengers regularly complain of expensive fares.

Haigh said that transitioning to public ownership would save money by cutting out franchise bidding costs, while still leaving a role for the private sector.

Her announcement was welcomed by trade unions and echoes a similar policy announced by hard-left former Labour leader Jeremy Corbyn.

But it was criticised by the government, which said that privatisation had led to a doubling of train users in 30 years.

Britain's state-owned railway system was privatised in the mid-1990s under former Conservative prime minister John Major.

But many operators have since come back under public control in recent years due to poor performance and operating costs.

The current Tory government has pledged to create a new public sector body to award contracts to operate trains but it would not be publicly owned.

Sunak is yet to announce the date of the election but it is expected to be held in October or November.

Labour, led by the centrist Keir Starmer, has enjoyed double-digit leads in most polls since Liz Truss was ousted as UK leader in October 2022.