It says that mail should be delivered to addresses six days a week, however local issues mean this may temporarily not be possible.
Royal Mail says a number of local offices are impacted, including Warrington.
High levels of sick absence, resourcing and other local factors are the reasons for the delays.
It said that the affected areas – including Warrington – will see deliveries be rotated so individuals see a minimal delay in receiving their mail.
Delays are being experienced for residents in WA1, WA2, WA4 and WA5.
The offices having issues will also receive ‘targeted support’ to try to ‘address their challenges’.
Postcodes that might not receive six days of mail a week from Royal Mail
So which postcodes are most affected? Here is the full list from Royal Mail.
- Altens DO (AB10, AB11, AB12, AB35)
- Ashington DO (NE22, NE62, NE63, NE64)
- Barry DO (CF62, CF63)
- Batley DO (WF5, WF15, WF16, WF17)
- Belfast West DO (BT10, BT11, BT12, BT17)
- Bingham DO (NG12, NG13)
- Bristol East DO (BS5)
- Brixton DO (SW2)
- Calderway DO (WF12, WF13, WF14)
- Canvey Island DO (SS8)
- Clevedon DO (BS21, BS49)
- Congleton DO (CW4, CW12)
- Dundee East DO (DD4, DD5, DD7)
- Dunstable DO (LU5, LU6)
- Dyce DO (AB21)
- Ebbw Vale DO (NP23)
- Erskine DO (PA7, PA8)
- Glenrothes DO (KY6, KY7)
- Gloucester North DO (GL3, GL4)
- Gravesend Rural DO (DA12, DA13)
- Grimsby DO (DN31-DN37)
- Hyde DO (SK13, SK14, SK16)
- Inverclyde DO (PA14-PA19)
- Inverurie DO (AB51, AB52)
- Johnstone DO (PA5, PA6, PA9, PA10, PA12)
- Kettering DO (NN14-NN16)
- Kidsgrove DO (ST7)
- Kilmarnock DO (KA1, KA2, KA3, KA4, KA16, KA17)
- Lichfield DO (WS7, WS13, WS14)
- Lisburn DO (BT26, BT27, BT28)
- Lochgelly DO (KY5)
- Londonderry DO (BT47-BT48)
- Mid Rhondda Ganol DO (CF39-CF40)
- Motherwell DO (ML1)
- Northfield DO (B31)
- North Tyneside DO (NE25-NE30)
- Northwich DO (CW8-CW9)
- Nottingham South DO (NG2, NG12)
- Ongar DO (CM5)
- Plymouth DO (PL1, PL4, PL6, PL8, PL9)
- Plympton DO (PL7)
- Pontefract DO (WF7, WF8, WF9, WF11)
- Prescot DO (L34, L35)
- Radstock DO (BA3)
- Renfrew DO (PA4)
- Sleaford DO (NG34)
- St Helens DO (WA9, WA10, WA11)
- Syston SPDO (LE7)
- Thornbury DO (BS35)
- Tipton DO (DY4)
- Uddingston DO (G71)
- Warrington DO (WA1, WA2, WA4, WA5, WA55)
- Wellington DO (TA21)
- Westhill DO (AB13, AB14, AB32)
- Whitehaven DO (CA18-CA20, CA26, CA28)
- Whitwood DO (WF6, WF10)
- Wigan DO (WN1-WN6)
- Yate DO (BS37)