Such a default would raise the spectre of default by other over-indebted eurozone governments, which in turn would undermine the perceived solvency of some very big banks.
Dave Prentis, general secretary of public service union Unison accused the coalition government of "declaring war" on public services, saying the Budget would "raise the spectre of breadline Britain" in some parts of the country.
It raise the spectre of foreign, trade and defence ministers being grounded in the capital, and of Ms Gillard's overseas trips to be limited to major meetings of world leaders or visits to major trading partners, our correspondent adds.