The unofficial industrial action by workers from Oxford's main sorting office began on Tuesday night.

Royal Mail and the Communications Workers Union (CWU) differ on how many staff are taking part in the strike.

A statement released by the Royal Mail and the union said there had been "tensions within the office" between staff members.

Both parties said they were "concerned by the unrest within the office at the current time".

A joint statement signed by the mail centre manager at Royal Mail Oxford and Bob Cullen, of the CWU, said: "We expect all staff to maintain the standards set down within Royal Mail's code of conduct and to treat each other with respect and dignity.

"Over the last week, there have been heightened tensions within the office.

"Staff must be able to come to work in an environment where they can go about their duty, peacefully and safely."

An investigation looking at the shift patterns is expected to get under way at the end of the week.

The inquiry will be carried out by an independent external company.