London - Incoming British Prime Minister Boris Johnson will take power with an array of issues looming large -- from resolving Brexit amid growing uncertainty to managing an unfolding geo-political crisis with Iran.
Here are the main challenges confronting the new British leader as he enters Downing Street:
- Brexit -
Britain's stalled departure from the European Union, which forced Johnson's predecessor 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 May struck with Brussels, but European leaders have shown little willingness to make new compromises.
Johnson also insisted Britain will leave the bloc on the latest delayed deadline of October 31 without an agreement if necessary.
However, a majority of British MPs -- including many within his own Conservative ranks -- bitterly oppose a so-called no-deal departure, leaving 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, Johnson will have to deal with the expected disruptions to trade and wider economic fallout.
Boris Johnson vows to 'get Brexit done' on October 31
- House of Commons support -
Johnson's Conservatives do not have an outright majority in parliament and rely on an arrangement with Northern Ireland's Democratic Unionist Party (DUP) to pass legislation and remain in power.
But with opposition MPs gunning for the new leader and some europhile Tory lawmakers uneasy about his 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 Johnson attempts to force through a disorderly departure from the EU in October -- by suspending parliament or another means -- and could ultimately leading to a general election.
A few dozen Conservative MPs, including prominent Cabinet members under 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 -
Johnson takes power as relations with Iran remain in a heightened state of tension amid a standoff between the two countries over the seizure of tankers.
Iran's Revolutionary Guards seized a UK-flagged ship in the strategically vital Strait of Hormuz on Friday in apparent retaliation for British authorities detaining an Iranian vessel on July 4 in the Mediterranean.
Iranian Foreign Minister Mohammad Javad Zarif warned Johnson Tuesday that the country will "protect" waters of the oil-rich Gulf, while insisting it "does not seek confrontation".
The flare-up comes amid rising hostilities between Iran and the United States, after US President Donald Trump withdrew from the 2015 deal aimed at limiting Tehran's nuclear programme and began reimposing sanctions.
UK-flagged tanker seized by Iran
- Economic uncertainty -
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 ($37 billion, 33 billion euros) per year from 2020 on sliding tax receipts, should London default to the World Trade Organization (WTO) tariff system.
Meanwhile the International Monetary Fund said Tuesday the world economy faces a "precarious" 2020, citing the rising prospect of no deal as well as global trade tensions.
What will a no deal Brexit mean for Britain and the EU? (slideshow)
By Joe Jackson