Up to 3,500 Royal Mail staff may walk out after members of the Communications Workers Union (CWU) backed industrial action by four to one.

No strike dates have yet been set, but the union said it would maintain its walkout threat until it received reassurances that no compulsory redundancies would be imposed during the closure of two centres in south and east London, and “concerns of bullying are addressed.”

CWU divisional official Martin Walsh said: “London postal workers have sent a clear message to Royal Mail in this ballot that they will not be bullied or intimidated by the company.

“Royal Mail’s closure plans are a clear threat of compulsory redundancy and this is completely unacceptable. In their race to push services to the bottom Royal Mail will eventually provoke a reaction wider than London.”

But Royal Mail said that the closure of mail centres at Vauxhall and Bow had been agreed during a consultation with the CWU. It insisted that the 580 redundancies would be conducted on a voluntary basis, and that nearly 700 full-time staff had already expressed an interest in accepting severance packages.

“There is no justification for strike action.The reality is that almost 700 full-time Royal Mail people in London have already expressed an interest in taking a voluntary redundancy package that is worth up to two years’ salary. More than 300 are already being progressed.”

CWU members were balloted at four workplaces in the capital – Nine Elms mail centre in Vauxhall, East London mail centre in Bow, Mount Pleasant mail centre in central London and the delivery offices at Rathbone Place.

Royal Mail announced plans to shut the mail centres in Vauxhall and Bow earlier this year, blaming falling mail volumes in London, which are predicted to halve by 2014.