I resolved this issue but it took a while.
I contacted BT and they decoupled my btinternet email address from Yahoo. This meant I had to create a new Yahoo account and recreate my portfolio from scratch. I could not find any other way around it. I tried all sorts of things the folk on here recommended and nothing worked.
It was BT that resolved the problem for me - not Yahoo - they decoupled my btinternet account from Yahoo.
I understand that, but then you had to set up a YAHOO account to access you YAHOO FINANCE PORTFOLIOS and rebuild the portfolios.... YES???. I want to access 5 year's of portfolios with my still existing @btinternet.com username, YAHOO is preventing my from doing this. I wish people would read these posts and stop replying to my problem with answers that have nothing to do with the previous problems. There has been no solution to this redirection problem which only started occurring after BT updated their email but YAHOO did not update their redirection to a BT Login page for @btinternte.com users.
I have merged the two threads that were running so that everything is in one place instead of repeating messages and jumping between two threads.
Thanks for that Distinguished Sage but until Yahoo actually read this thread or BT put them wise to it I cannot see a solution and Yahoo will certainly lose me as a customer. I have wasted a day on it and will now simply wait a few days for an answer from someone, but it really only YAHOO who can sort it by giving former BTInternet customers access to their Portfolios instead of simply using the quick fix of redirecting them. I cannot rewrite their access code for them but from friends who work in IT it is about a 10 minute fix by someone with access to the Yahoo code. When Corporates who were once working together fall out, their customers are the losers. I have a good mind to dump both YAHOO and BT after 30 years... so much for the end of BTYahoo. I do not intend to waste any more time on this I will simply use Simply Wall Street (which I use to analyse my portfolios anyway) as my Portfolio Tracker in future. Not as convenient or a means of recovering the last 5 years of Historic Portfolios, but I suppose COVID is destined to give is all a rethink and a fresh start
I use Chrome as my search engine and your remedy doesn't work for me I'm afraid