Royal Mail staff are racing to clear the backlog of undelivered items

More than 400 Royal Mail workers at the centre in Cowley and two delivery offices were involved in the industrial action, which ended on Thursday.

Keith Slater, president of Oxfordshire chamber of commerce said the effect on business has been dramatic.

Royal Mail is aiming to clear the backlog by the middle of next week.

Mr Slater said: "A combination of the estimated cost comes from banks' account charges with cheques not arriving, lost orders, plus people having to go out of the county to collect and get post delivered."

Workers are racing to clear about five million undelivered mail items, which have been piling up since the unofficial strike began on 30 March.

"It will also take time to clear the backlog so it will be at least seven days before businesses are back to normal," said Mr Slater.