BRITAIN's Royal Mail declared mail due for delivery is delayed as 115,000 postal workers are on strike, the biggest since Britain's summer of labour unrest, reports Bloomberg.

The walkout means regular post won't be processed until mailings from the National Health Service and other public bodies will be given priority.

Items sent using the Special Delivery and Tracked24 services should still arrive on time as 4,000 middle managers were drafted to maintain some deliveries.

Royal Mail is Britain's largest private-sector employer. The strike over pay and restructuring proposals involved twice as many people as recent strikes on the rail network.

Three more daily walkouts are planned, one next Wednesday and two the following week.

Royal Mail has granted staff a two per cent raise and is offering a two per cent productivity bonus plus 1.5 per cent based on an agreement on changes to the business.

Royal Mail CEO Simon Thompson declared he's ready to discuss a higher settlement if the union agrees to significant reforms.

"What we've been trying to do for over four months is to talk about the change we need to win in the market and to pay our team more," said Mr Thompson.

"We have working practices that are based on our letters history, and we need to change those so that we can compete in the parcels market."