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The damage caused by England's wettest January since 1766 has left Cameron battling to defend his government's response against criticism it did not do enough to prevent flooding in the first place and then was too slow to help those affected.
"Money is no object in this relief effort, whatever money was needed for it will be spent," he told a news conference in London, after spending 24 hours visiting flood-hit areas in south west England, where major rail links have been destroyed.
Cameron, who has taken control of the crisis after an escalating blame game between ministers and the Environment Agency, said he had no choice but to shelve a visit to Israel and Palestine scheduled for next week.
"I'm sending my apologies today to (Israeli) Prime Minister (Benjamin) Netanyahu and (Palestinian) President (Mahmoud) Abbas, but nothing is more important than dealing with these floods," he said.
Britain's two-party coalition government has faced increasing pressure over the situation, with critics saying problems have been exacerbated by years of underinvestment in river dredging and flood defences.
Cameron described the government's response to the flooding as strong and rejected calls to divert money from Britain's foreign aid budget to help victims of flooding.
"We don't have to make that choice. We are a wealthy country with a growing economy, with public finances that are increasingly coming under control," he said.
"We will spend the correct money here at home and we will do that without interfering with our aid budget."
Earlier on Tuesday, Defence Minister Philip Hammond was berated by a local resident during a visit to flooded Wraysbury, close to London, who said the authorities were not doing enough to help those affected by flooding.
Hundreds of homes along the banks of the River Thames west of London were evacuated on Tuesday. With water levels still rising and more rain forecast for the rest of the week, Cameron warned things may get worse before they get better.
In the badly-affected Somerset Levels area in south west England, more than 65 million cubic metres of flood water is being pumped out at a rate of 3 million cubic metres a day. The Met Office said 16 severe flood warnings, indicating a danger to life, remain in place across the south of England.
Cameron said the military, who have already been brought in to help build flood defences and evacuate homes, could play a greater role and said thousands more were on standby to be drafted in as needed.
"My focus is on the operational response helping those people who need help and protecting those properties which need protecting, it will be a long haul," he said.