People could be charged up to £3.50 to get their post when Royal Mail axes stamps without barcodes.
The postal service introduced barcoded stamps last week.
And people now have a year to use their regular non-barcoded stamps before they are axed on 31 January 2023.
From then, letters and parcels bearing the old ones will not be delivered.
They will be left at sorting offices with a “surcharge” – currently £2 a letter or £3.50 for a small parcel – to retrieve them.
Campaigner Malcolm Booth said it would “make life most difficult for the elderly and vulnerable – the very people who use stamps the most”.
He added: “Stopping people using normal stamps and making them pay to receive post will cause disruption.”
People sent items with the old stamps will receive a “fee to pay” card and be told to collect them from a sorting office.
But older people and those in rural areas – often miles from a mail depot – face long journeys to get post.
Royal Mail – which is battling week-long delivery delays in some areas – says Brits will be able to swap traditional stamps for the same value of new barcoded ones from March 31, via Freepost until 31 March 2023.
But campaigners warn some people will miss the announcement and only find out about the ban when it comes in.
Mr Booth added: “Many people have a stash of stamps at home. How many will not get the message about the changeover?”
Experts are calling for a switch-over period of several years where people can use both kinds of stamp.
A Royal Mail spokesperson said: “Non-barcoded stamps can be used as postage up until 31 January 2023.
"We encourage our customers to use them within this time but if they can’t, they can send them in to us and we'll swap them out for barcoded stamps free of charge.
"Our 'Swap Out' scheme will open on 31 March 2022 and they can pick up a 'Swap Out' form from their local Delivery Office's Customer Service Point; print one off our web site; or contact Customer Experience and one will be posted one out to them.”
