In the last quarter of 2009 less than 80 per cent of first class letters arrived the next day, this compared badly to the first quarter when 94 percent arrived the next day.
This report being in the Torygraph it stresses that the strikes, together with management failure to plan for the disruption, were to blame for the performance failure.
During October's dispute almost 60 million parcels, packets and letters clogged up sorting offices.
Nigel Woods, of Consumer Focus, said: "Whether or not these results are down to industrial action, consumers were let down by Royal Mail last autumn.So, to summarise, the stike caused the disruption which wouldn't have been as bad if management had arranged a massive scabbing operation.
"The figures are similar to those recorded during the industrial action of 2007-8 and show how important it is for Royal Mail to resolve their industrial relations problems once and for all.
"It also shows that Royal Mail's contingency plans have not stopped severe service disruptions from taking place during strike action.