Conservative failures to invest in flood defences will put an extra 330,000 properties at serious risk by 2035, Labour will claim on Tuesday. The Tories counter that they have actually cut the predicted number of homes at risk since Labour was in power. The row has emerged as the Environment Agency issues new flood warnings and Britain is lashed by the tail end of Hurricane Bertha.

Both major political parties recognise that the issue is key in a number of battleground marginal seats, and that the risk of flooding is rising.

Maria Eagle, the shadow environment secretary, says the figures emerged in a report to the Committee on Climate Change. By comparing long-term investment needs from June 2009 with spending plans up to 2021, the data shows that there has been a real cut of around 20% in funding, she will say.

"The choice facing the country at the next election is between a Conservative Party that has broken its promises to take climate change seriously and has no answers to the huge challenges we face and the Labour party, who offer a changed approach – one where we protect our climate for families, their homes and businesses," she will tell a meeting at the World Wildlife Fund's Living Planet centre.

A Conservative party spokesman has accused Labour of political posturing, saying that the future risk assessment has actually improved since May 2010.

"Contrary to what Labour will tell you, Britain has seen real improvements in its flood defences these past four years. This hasn't happened by accident. Because this government has taken difficult decisions on the deficit – and cut wasteful spending elsewhere – we've been able to spend more on flood defences, helping families plan for the future with confidence," the spokesman said.

The Tories have come under sustained criticisms for failing to fund defence spending properly. Caroline Spelman, the environment secretary in 2010, authorised cuts of £100m a year. Her successor, Owen Paterson, was given a further £120m to spend, in November 2012 – but only after hundreds of homes were flooded in south-west England, the Midlands and Wales.

This is not the first time that the parties have clashed over flood defence data.

In February, Sir Andrew Dilnot, the head of the UK Statistics Authority, contradicted government claims that it is spending a record amount shoring up defences against floods.

Following a complaint by Labour MP Hugh Bayley, Dilnot said his staff had looked into the matter and concurred with the Commons library figures. "We agree with their [Commons library] finding that, as at January 2014, government funding for flood defences was expected to be lower in both nominal and real terms during the current spending period than during the last spending period," he said.