Postal workers have begun three days of strike action in London in a dispute over jobs and pay, amid claims of cutbacks to services.
Delivery staff will strike on Wednesday, distribution workers on 9 July and mail centre staff on 10 July.
The Communication Workers Union (CWU) accused Royal Mail of "sulking" because privatisation plans have been halted.
But bosses said the union was "halting existing efforts to modernise Royal Mail in London".
Dave Ward, the Deputy General Secretary of the CWU, said Royal Mail had not taken up an offer of a three-month moratorium on industrial action in return for discussions on modernisation.
'Management sulking'
He added: "It's now clear that Royal Mail management is the biggest block to modernisation.
"Management seem to be sulking because the prospect of privatisation and increased executive pay has disappeared.
"Strike action in London is in response to Royal Mail's continuing executive action of cuts without modernisation."
Royal Mail spokesman wrote:Customers will not understand how the union can claim to support modernisation but then announce strike action to halt existing efforts to modernise Royal Mail in London
But a spokesman for the company said: "Royal Mail wants to discuss how to fully implement the agreement in London, as has already happened in units in the rest of the UK - the union refuses point blank to do so.
"Customers will not understand how the union can claim to support modernisation but then announce strike action to halt existing efforts to modernise Royal Mail in London."
The Business Secretary Lord Mandelson announced last week that plans to sell a minority stake in Royal Mail had been put on hold, blaming the recession and a lack of parliamentary time to pass the relevant legislation.
And speaking before the business select committee on Tuesday he accused the union of "boycotting" agreed procedures for implementing changes.
Up to 8,000 CWU members in London and about 160 in Scotland held a two-day strike on 19 June as part of the same dispute.