It is poised to begin trading as a FTSE-100 company on December 23, following a share surge that has seen its value increase to nearly £6 billion.
The stock was initially offered at 330p before climbing strongly - sparking accusations that it had been sold off on the cheap by Business Secretary Vince Cable.
It closed tonight at 596.5p, meaning it looks certain to join the top-flight when the FTSE Group finalises its reshuffle, which will be confirmed tomorrow.
Chief executive Moya Greene will now become one of just four female FTSE-100 bosses, joining easyJet's Carolyn McCall and Imperial Tobacco's Alison Cooper.
A fourth, Angela Ahrendts of Burberry, is leaving next year but BT executive Liv Garfield will make up the number when she takes over at Severn Trent.
The FTSE Group earlier published the names of the firms whose positions at the close on Monday indicated that they looked likely to move
The confirmed reshuffle will be based on the market value of companies based on tonight's closing share prices.
Also moving into the FTSE-100 should be construction and industrial equipment rental firm Ashtead, with chemicals group Croda International and miner Vedanta Resources relegated from the shares premier league.
Tipped for the second-tier FTSE-250 were Madame Tussauds and Alton Towers owner Merlin Entertainments and estate agency Foxtons, following their own recent flotations.Merlin's market debut in September saw it offered at 315p and it has now reached 352p, 12% ahead.