More than a dozen postal workers have staged a walk-out at the town centre’s Royal Mail office.
The protest is understood to centre on pay and conditions.
It is believed around 15 workers were involved in the strike yesterday morning (Thursday).
A Royal Mail spokeswoman said: “Royal Mail is disappointed that unballoted industrial action is taking place today at St Helens delivery office. This covers the WA9, WA10 and WA11 postcode areas.
“We sincerely apologise to customers in these areas for any disruption to their mail services.
“We are doing all we can to minimise this disruption by bringing in extra resources from the neighbouring area. The customer service point is open as usual for customers to collect items for which we have left a ’Something for You’ card.”
She added: “Mail collections from postboxes in the area may also be affected this afternoon so customers are advised to post mail outside of the area, drop mail off at any main Post Office branch or at the delivery office (23 Liverpool Road, St. Helens, WA10 1AA).
“Royal Mail is fully committed to the process to resolve concerns of postal workers based in the St Helens office by following the national jointly agreed framework with the Communication Workers Union (CWU).”