The stock fell 101p to 829p, its worst single-day fall since 2008, while BT edged up 2p to 374.1p.
ITV, which also lost its grip on European action, fell 2.8p to 187.7p as Citigroup removed its “buy” recommendation.
Trevor Green, head of institutional equities at Aviva Investors, said: “BT has come out with this deal today and, even by doing this, managed to keep its existing guidance so that’s positive.
“It could be up to three years before one can say definitely the price they paid was justified. It’s what we expected and we’re comfortable with it.”
The wider FTSE 100 index climbed 19.95 points to 6,728.37, boosted by BT’s deal and drugs maker Shire winning the £2.6 billion race to buy American peer ViroPharma. Shire climbed 26p to hit 2,822p, an all-time high for its stock.
Elsewhere in the top flight, shares in the state-backed banks rose after details emerged of the first month’s take-up for the UK government’s extended Help to Buy scheme.
Royal Bank of Scotland said it has taken 1,075 applications, with almost three-quarters of customers looking to buy their first home.
Halifax, which is part of Lloyds Banking Group, has received 1,309 applications during a “really strong start” to the scheme. Lloyds rose 1.6p to 76.8p and RBS gained 17.3p to 339.8p.
Shares in Aberdeen-based driller Ithaca Energy jumped by 5.8 per cent or 8.25p to 151.75p after the Aim-quoted explorer unveiled record quarterly profits of $54.1m (£33.8m) despite some of its facilities shutting down for planned maintenance.