Royal Mail has apologised for a third successive year of poor service delivery.
Wildcat strikes have caused a backlog of mail deliveries
Chief Executive Adam Crozier said wildcat strikes and a radical structural overhaul were to blame.
Industry watchdog Postwatch believes the organisation is unlikely to have met any of its licence service standards in the last financial year.
Business analysts believe regulator Postcomm will now levy millions of pounds in fines.
'Not nearly good enough'
Speaking to BBC Radio Five Live's Weekend Business team, Mr Crozier admitted there had been problems.
"The first thing to say is a huge apology for not providing the service that we should," Mr Crozier said.
"For the total year we will probably have failed our annual target by around 2.5% which is not nearly good enough."
Mr Crozier's words come as Postwatch has reported dramatic increases in complaints about missed and late deliveries, as well as the loss of the second daily delivery.
Promising eventual improvement Mr Crozier told the BBC's Jeff Randall Royal Mail had undergone more change in six months than in over 30 years.
But he defended the move to introduce a single daily delivery service.
"Single delivery is taking a few weeks to bed in, but it does seem to take a little more time than we expected," he said.
"We need to make this change because if we don't we are simply not going to be able to compete with other people coming into the market place in future."
Restructuring at Royal Mail has already helped the company make an estimated £200m profit last year.
Postmen's collection routes are being changed and it is reshaping its national transport distribution business.
But a major efficiency programme led to wildcat strikes in London last year which in turn created a backlog of deliveries.
And recent strikes in Oxford may have cost businesses nearly £1m, according to the county's chamber of commerce.