Royal Mail managers have said post has been arriving on Oxford doorsteps without a frank mark because of the large volume of mail from the census.

Big envelopes have to be franked by hand and, to speed up the process, some were omitted for a few of days at the end of last month. The Royal Mail said it would now investigate why post received by the Oxford Mail on Wednesday did not contain a frank mark.

There has also been criticism about the time it takes mail to be delivered. A Royal Mail spokesman said: "For mail posted in Oxford for Oxford addresses we have delivered 94 per cent of first-class mail the next working day."