Letter deliveries had been on target

It said the staff walkouts had put at risk next March's annual delivery targets.

Before the wildcat action, first-class letter reliability had reached 92.7%, the best performance for 15 years.

Adam Crozier, the Royal Mail's chief executive said it was a "frustrating setback" after such strong figures.

"The vast majority of our people, around 80%, worked normally during the unofficial action," Mr Crozier added.

"But the disruption in the south east and some other parts of the country had an impact on our whole network."

Reform

Mr Crozier also warned that the full impact of the strikes on the Royal Mail's quality of service performance figures were not yet known, but would be "significant".

For the three months to September, around 98.7% of second-class mail was delivered on time, against a target of 98.5%, the Royal Mail said.

Earlier this month, the Royal Mail unveiled its first profit for five years, posting a pre-tax profit of �3m for the six months to September.

But the period did not include the period affected by the unofficial walkouts, the cost of which has been estimated at about �40m.

The Royal Mail is shedding thousands of jobs and wants to change working practices as it seeks to increase its profitability.