The Skipton Building Society will withdraw its 5% fixed rate ISA at the end of today.
Launched on 2 March, the ISA spent two weeks at the top of the best buy tables.
Eager savers have until 5pm to snap up the deal. It is fixed at 5% for five years and is available with a minimum investment of £500.
The ISA also accepts transfers from previous years' ISAs and savers can opt to receive interest monthly (instead of yearly) for a slightly lower rate of 4.89%.
Skipton says they will replace the ISA with another deal later this week but no details have yet been confirmed.
Whichever interest option savers plump for, the deal is well ahead of the competition. The next best long-term fixed ISAs are 4.3% for five years from Northern Rock, or 4.3% for four years from Halifax and Royal Bank of Scotland.
With the base rate still languishing at 0.5%, any account that pays 5% is a tempting option, however the Skipton deal won't be suited to everyone. Savers need to be sure they can afford to tie their money up for the full five years.
Should you need to get your hands on the money before that time you would suffer a penalty of 240 days interest, have to withdraw the total balance and close the account.