The mail operator announced last week that it was not going to use rail in future because the cost was too high.
Paul Bateson, Royal Mail's managing director, logistics, said: 'There is a marked difference between the price we believe we should be paying for rail services and that which was on the table. Other forms of transport can give us the same benefits, in terms of flexibility and quality, but at a lower cost.
'We need to move ahead and create a new distribution network which is more robust and has greater flexibility to improve quality of service as well as one which is more cost-effective than we have now.'
EWS chief operating officer Allen Johnson said: 'We are shocked that Royal Mail is threatening to walk away from the railway. EWS will continue to press the many benefits of rail to Royal Mail.'
A Royal Mail spokeswoman said the decision was unlikely to cause extra congestion as it would not be increasing the number of vehicles it uses. Instead, the company will make better use of vehicles it already has.
Bateson said the company had not ruled out a return to rail for some elements of distribution and that rail companies could be invited to tender.