The $1.2tn in automatic spending cuts that Barack Obama once promised to avert are looking increasingly likely to occur because of entrenched politics in Washington, threatening a shock to confidence in the US economy.

Economists have long assumed that the so-called sequester – a budgetary mechanism passed in 2011 that takes effect on March 1 and slashes the Pentagon’s budget by $600bn over 10 years while cutting discretionary spending for government programmes by another $600bn – would be replaced or reversed by Congress.

US faces fresh financial shock