What is the highest hit point that one character can have?

By reading others questions like Highest damage in one melee attack and Highest AC possible.

How many hit points can a character have, and also highest achievable hit points possible in an instant?

Conditions :

We’re looking at a level 20 character and no multiclass.
Feats are allowed.
Magic items are those on the tables in the D&D 5th Edition DMG.
Spells, skills, feats and abilities available to a player character are eligible for use/consideration.
No other help from an ally.