The Royal Mail is delivering more first class letters on time, but it says there is room for improvement.

More than nine out of ten first class letters arrived a day after being posted between July and September, according to new figures.

This was a 1% improvement on the same period last year. Second class mail performance was 98.6%.

But the postal watchdog Postwatch said the figures could not disguise that a quarter of postcodes, many in densely populated urban areas, are still receiving a "sub-standard" service.

Jerry Cope, the Royal Mail's managing director, accepted more improvements are needed.

"The results are going in the right direction but too slowly and without enough consistency across all services and all parts of the UK," he said.

The first class figure of 91.5% was still 1% behind the company's target for the end of the current financial year.

Regional variations

In London, first class was slower than the national average at 90.1% - but an improvement on last year's figure of 85.7%.

But some London postcodes have seen little improvement, according to Mr Cope.

Action plans had been set up for those areas, he announced.

Postwatch's chairman Peter Carr said: "It is almost impossible for Royal Mail not to meet the target imposed in its licence for a minimum delivery standard across all postcode areas for the entire year."

He called on the industry's regulator to act against this "potential breach" of the Royal Mail's licence.