Charity shops are set to reopen from June 15, although all donations will have to be quarantined for 72 hours.
English stores fear they will be inundated with items when they reopen due to people finally getting time to have a clear-out during the lockdown.
Outlets expect three months' worth of donations on top of the goods people have collected while de-cluttering their homes since March.
However, shops will isolate goods for 72 hours before sorting them as part of new coronavirus measures.
At least one charity says it will only commit to reopening once it is "safe to do so".
Robin Osterley, chief executive of the Charity Retail Association, told the BBC : "We're not just anticipating a normal three months' worth of donations but also the extra stuff that people may have picked out to donate during their clean-ups."
Oxfam intends to ask customers to stagger donations to stop items being left outside.
The charity said it is working on a 'detailed plan' to enable its shops to reopen but does not currently have a confirmed date, and pointed out that many of its 20,000 shop volunteers are elderly.
The charity said in a statement: "Our priority remains the health and wellbeing of our staff, volunteers and customers.
"Many of our volunteers are elderly and vulnerable and we are acutely conscious of our responsibilities towards them.
"We are following government advice and are taking comprehensive steps to create a safe environment, including ensuring social distancing within the shops, providing personal protective equipment, regular cleaning of all surfaces, doors and equipment, and the isolation of donated items for 72 hours prior to listing for sale.
"We welcome the fact that many people are taking the opportunity to de-clutter during lockdown and we are asking people to hold on to those items for now and donate them to Oxfam when the shops and donation banks are open again."
FARA said it will only be reopening its shops 'when each shop is safe to do so as our first priority is the safety of our staff and volunteers, customers and donors'.
Barnardo's is advising people to ring up stores before donating to check if they can take any further items.
It will be opening 70 of its shops in England from June 15 as part of a phased reopening plan and will have a range of safety measures in place.
Traid (Textile Reuse and International Development) is also planning to open some of its charity shops around June 18, beginning with the most spacious stores in Dalston, Kilburn, Peckham and Shepherd's Bush.
