Industrial action began at the city's main office in Cowley on Tuesday 30 March over claims of bullying.

Now workers at the sorting office in Headington have walked out in support of their colleagues.

Royal Mail managers have been drafted in from around the country to help sort the post at Cowley but there is still a huge backlog of undelivered mail.

We consider this a health and safety issue, the atmosphere was such that it was unsafe to work in Bob Cullen Union representative

Staff are refusing to accept an overtime ban managers are seeking to enforce on all those who have walked out.

Managers have defended the overtime ban as standard Royal Mail policy.

"Unofficial strikes are against the law", said the spokeswoman, "we don't feel we have any choice in the matter".

"If we didn't take action like this then we'd see this repeated elsewhere."

Costing business

Bob Cullen of the Communication Workers Union said: "It is totally unacceptable that they want to impose punitive measures on our members who walked out.

"We consider this a health and safety issue, the atmosphere was such that it was unsafe to work in."

Oxfordshire Chamber of Commerce has criticised the strike saying it has cost local businesses over �500,000.

Some firms are now planning to claim compensation from Royal Mail over the disruption.