Speaking to ITV News, the Royal Mail said:
We can confirm that a small number of postboxes in the Tooting area were sealed after a set of collection keys went missing.
The boxes were cleared of mail before they were sealed so no items were left inside them.
Royal Mail apologises to its customers in Tooting for any inconvenience this may have caused.
We are working to change the locks and reopen the boxes as quickly as possible. In the meantime, customers can use other postboxes in the area or of course drop their mail off at any Post Office branch.