BREXIT
Britain's stalled departure from the European Union, which forced Mr Boris Johnson's predecessor, Mrs Theresa May, from power, is the most pressing problem confronting him on day one.
He vowed during the leadership campaign to try to renegotiate the divorce deal Mrs May struck with Brussels, but European leaders have shown little willingness to make new compromises. Mr Johnson also insisted Britain will leave the bloc on the latest delayed deadline of Oct 31 without a deal if necessary.
However, a majority of British Members of Parliament - including many within his own Conservative Party - bitterly oppose a so-called no-deal departure, leaving Mr Johnson little room to manoeuvre if he is to make good on his pledge of delivering Brexit. If a no-deal Brexit ultimately does happen, Mr Johnson will have to deal with the expected disruptions to trade and wider economic fallout.
HOUSE OF COMMONS SUPPORT
The Conservatives do not have an outright majority in Parliament and rely on an arrangement with Northern Ireland's Democratic Unionist Party to pass laws and stay in power.
But with opposition MPs gunning for the new leader and some europhile Tory lawmakers uneasy about Mr Johnson's style, character and no-deal Brexit plans, attempts to unseat him through a vote of no confidence in his government could be on the horizon.
Such a vote would be more likely if Mr Johnson attempts to force through a disorderly EU departure in October - by suspending Parliament or other means - and could ultimately lead to a general election.
A few dozen Conservative MPs, including prominent Cabinet members under Mrs May, are openly trying to deny him the option of leaving without an agreement and some have hinted they would be willing to bring down the government to stop it.
IRAN CRISIS
Mr Johnson takes power as relations with Iran remain in a heightened state of tension amid a stand-off between the two countries over the seizure of tankers.
Iran's Revolutionary Guards seized a British-flagged ship in the strategically vital Strait of Hormuz last Friday in apparent retaliation for British authorities detaining an Iranian vessel on July 4 in the Mediterranean. The flare-up comes amid rising hostilities between Iran and the United States, after President Donald Trump withdrew from a 2015 deal aimed at limiting Teheran's nuclear programme and began reimposing sanctions.
ECONOMIC UNCERTAINTY
Mr Johnson takes charge with global growth forecast to slow and Britain's economy showing sluggish growth amid the continued Brexit uncertainty and the cloud of a potential no-deal departure from the EU.
The government's official forecaster warned last week that the country would slide into a year-long recession should it leave the bloc without a comprehensive agreement. It added that state borrowing would balloon by about £30 billion (S$51 billion) per year from next year on sliding tax receipts, should Britain default to the World Trade Organisation tariff system.
Meanwhile, the International Monetary Fund said on Tuesday that the world economy faces a "precarious" 2020, citing the rising prospect of a no-deal Brexit as well as global trade tensions.
AGENCE FRANCE-PRESSE