The union threatened to stage one lasting four days unless more talks in the pay and working practices dispute are held, a move denounced by the company as "outrageous".
The executive of the Communication Workers' Union said industrial action will be increased to affect deliveries from 30 August for four days if the talks are not re-opened by the time of the next scheduled strike on Thursday.
It disputed management claims that support among members for the strikes was weakening. Alan Johnson, the union's joint general secretary, said he hoped the four-day strike would not be necessary and promised that efforts would be made to resume negotiations with the Royal Mail.
The Royal Mail said it was "outrageous" to threaten further strikes while continuing to refuse to ballot members on an offer worked out during marathon talks at Acas.
Richard Dykes, the managing director, said: "We are appalled at the union's complete disregard for customers and for the views of its own members. Further strike action is not going to resolve this dispute. It will only make the situation worse.
"The union executive are burying their heads in the sand. Threatening further strikes will put the jobs of their members at risk as well as causing further disruption for customers."
The union was "ignoring reality" in claiming that support for the strikes was solid, Mr Dykes said, and added that 19,400 postmen and women worked during last Wednesday's strike, double the number who did so during the first walkout.
The union said however that support had been stronger in some regions. In a briefing paper to union branches, Mr Johnson said more effort would be made to resume negotiations.
Given the Royal Mail's efforts to claim that the dispute was crumbling and that workers were insisting on a ballot, Wednesday had been the "most crucial" date in the programme of industrial action, he said.
The Department of Trade and Industry made no direct comment on the executive's decision. A spokesman said the Government's one-month suspension of the Royal Mail's monopoly would be reviewed at the end of the period.Reuse content