John McDonnell has admitted that Labour's general election manifesto, which will be launched on Tuesday 16 May, is not fully costed.
The development came after the shadow chancellor was grilled by Nick Robinson on BBC Radio 4's Today programme over one of the party's flagship policies, nationalising the UK's water companies.
"We will seek to negotiate and we will look at the options of how we bring it back into public ownership," McDonnell said.
"One is, yes, an outright purchase, that could be done and we wouldn't want to set a price for that now because that would affect the negotiations."
He later added: "We wouldn't want to discuss any form of price at this point in time. Parliament, a result of past legal actions, has the ability to set the price in which any public ownership method like this will be designated."
McDonnell said the proposal would be in the public good since water prices have risen past inflation, partly because of privatisation, according to the shadow chancellor. But purchasing the utility firms is expected to cost the Treasury tens of billions of pounds.
Shares in FTSE 100 water firm United Utilities fell 1.5%, while rival Severn Trent slipped 1% in early trading.
Elsewhere, McDonnell defended Jeremy Corbyn's decision to appoint Andrew Murray, a former member of the Communist Party of Britain, to his general election team. "We are converting people to our cause of democratic socialism," the left-winger said.
The manifesto launch will come just over three weeks before the general election on 8 June. The latest poll from ICM, of more than 2,000 voters between 12 and 14 May, put the Conservatives 20 points ahead of Labour (48% versus 28%).