The FTSE 250 fell 0.4%, its worst day in more than a week. The FTSE 100 ended roughly flat, with notable gains in plumbing products distributor Ferguson and packaging firm DS Smith.
Stocks on the domestically-focused mid-cap index as well blue-chip local financial shares skid after Johnson, the face of Britain's campaign to leave the European Union in the 2016 referendum, was backed by more than a third of voting lawmakers.
This again raised the risk of Britain crashing out of the EU without a deal on Oct. 31, which Johnson has said he would do if no agreement had been ratified by then.
Ferguson surged 6.1%, its best day in more than a year after activist investor Nelson Peltz's Trian Fund disclosed a stake and said it would work with the company, formerly known as Wolseley, to explore creating long-term value for shareholders..
DS Smith jumped 5.4% after it posted solid pre-tax profit growth, helped by higher volumes, sales prices and margins. It dragged shares of peers Smurfit Kappa and Mondi higher.
Companies trading ex-dividend were the biggest fallers. Hosebuilder Persimmon and ad firm WPP gave up more than 4%.
Tesco recovered from losses earlier in the session to end 0.9% higher. Britain's biggest retailer had flagged slowdown in underlying sales growth in its latest quarter.
"Whilst no one thought the Christmas boost would carry through completely into the first quarter, these are mildly disappointing results. The threat from discounters is not going away," Markets.com analyst Neil Wilson said.
Mid-cap Just Group climbed 12.7% on its best day since December after the specialist pension provider's chairman said his focus was on maximising shareholder value with "no options excluded".
AIM-listed Majestic Wine slumped 8.2% after it suspended its dividend due to the sale of its retail business and said Chairman Greg Hodder would resign.
Liberum analysts called Hodder a highly regarded executive and said his departure was unexpected.
(Reporting by Shashwat Awasthi and Muvija M in Bengaluru; Editing by Saumyadeb Chakrabarty and John Stonestreet)
By Shashwat Awasthi and Muvija M