Postwatch is issuing warnings and guidance to customers ahead of Friday's planned 24-hour postal strike.

The industrial action begins at 3am and there will also be two six-hour strikes in Crown post offices starting at 6am on Friday and Monday.

Postwatch has stated the vast majority of post offices - the sub-post offices - will not be affected and will be open as normal. The Royal Mail will also use managers to keep the Special Delivery service running as best they can.

They will not, however, offer the usual money back guarantee of delivery by a specified day and time. This guarantee will be suspended for items posted both tomorrow and Friday.

The impact of the strike will, to a large extent, depend on the scale of the action, but customers should assume they will not receive mail on Friday and that mail for a few days afterwards might take longer to arrive as backlogs in the system are worked through.

Royal Mail plans to continue emptying some post boxes and collecting from post offices, although workers ask that customers help to reduce strain on this skeleton collection service by not posting from tomorrow through to Monday .

Customers expecting important mail should contact senders to discuss ways of getting around the postal problems - for example, by finding out how much is owed on a credit card and organising payment so as to avoid extra charges.

Royal Mail will operate a helpline 08457 740 740 for customers.