The postal workers strikes have led to 30 million letters being delayed, it was revealed today.
The Royal Mail said the figure was equivalent to around 40 per cent of an average daily postbag.
The company branded the strikes as "unnecessary and irresponsible", as a second day of action crippled deliveries.
Meanwhile the Communication Workers Union confirmed there will be a three day strike from next Thursday involving over 120,000 workers.
A Royal Mail spokesman said: "We are very grateful to the 20 per cent of our delivery staff who have chosen to come to work today and who are doing everything possible to get all delayed mail delivered to customers as quickly as possible over the next few days."
The union said it was reiterating its offer of unconditional talks at the conciliation service Acas to try to break the deadlocked row over pay, jobs and modernisation.
Deputy general secretary Dave Ward said: "We have six days before any further strike action would take place. Given the progress we were making in talks earlier this week, this should be enough time to reach an agreement.
"We want to go to Acas with no preconditions on either side to resolve this dispute."
Next week's 24-hour strikes will involve 43,700 staff across the UK in mail centres, delivery units in mail centres, network logistic drivers and garage staff on Thursday; 400 workers in Plymouth, Stockport and Stoke who deal with poorly-addressed mail, on Friday; and 77,000 delivery and collection workers on Saturday.
Lord Mandelson today denied union claims that he was orchestrating the postal strike.
The Business Secretary described CWU's accusations as "nonsense from beginning to end".
The union said today's strike by 78,000 delivery and collection workers was "solidly supported".Reuse content