The US moved a step closer to a government shutdown last night, as the Republican-held House of Representatives passed a measure to defund Obamacare, which Democrats in the Senate swiftly vowed to reject.

In a late-night session, House Republicans pushed through an amended version of the spending bill by 231 votes to 192, which stripped funding from President Obama’s Affordable Care Act for the coming year. Yet Democrat Senate Majority Leader Harry Reid insisted he and his colleagues would vote down the bill when it arrives in their chamber tomorrow afternoon.

If Congress cannot agree on a spending bill before the new fiscal year begins on Tuesday, the US government will run out of money to fund its non-essential services, leading to its first shutdown since 1996. Accusing Republicans of “futile political games”, Mr Reid said the Senate would “reject any Republican attempt to force changes to the Affordable Care Act through a mandatory government-funding bill... The American people will not be extorted by tea party anarchists.”

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Before the House vote on Saturday, the White House warned that President Obama would veto any measure that disrupts his signature policy achievement, which is intended to provide health coverage to millions of uninsured Americans. The White House spokesman Jay Carney told reporters, “Any member of the Republican Party who votes for this bill is voting for a shutdown.”

Should the federal government shut down on Tuesday, around 800,000 of its 2.1 million employees would be forced to stop working, though essential services such as air traffic control would be maintained. Furloughed workers would have no guarantee of back-pay should the shutdown be resolved later.

Even if a shutdown is averted, Washington faces a further battle over the raising of the debt ceiling: unless Congress approves an increase in its borrowing limit, the US Treasury will be unable to meet its debt obligations from 17 October.