Source: Sharecast
The FTSE 100 was called to open around 0.2% lower.
Ipek Ozkardeskaya, senior analyst at Swissquote, said: "Yesterday, the US announced it would escort ships through the Strait of Hormuz - calling it a humanitarian operation (!) Iran warned it would retaliate, and reports suggest the US went ahead regardless.
"Iran then struck ships and key oil infrastructure in Fujairah, UAE - notably a terminal that allows exports bypassing the Strait. Unsurprisingly, oil prices spiked, with US crude nearing $110pb and Brent $115pb. This morning, uncertainty remains high, though prices have eased slightly."
At 0720 BST, Brent crude was down 0.8% at $113.57 a barrel.
In corporate news, HSBC posted a surprise decline in quarterly earnings despite a spike in net interest income, on the back of higher credit impairments.
Pre-tax profits at the lender fell to $9.4bn from $9.5bn a year previously, missing forecasts for around $9.6bn.
HSBC said expected credit losses had risen by $0.4bn to $1.3bn during the first quarter, following a loss relating to British fraud case, geopolitical tensions and higher trade tariffs.
Plus500 said it remains confident in its outlook for FY26 following a better-than-expected performance in the first quarter.
In a statement ahead of its annual general meeting, the company said it entered the year with strong momentum across both its OTC and non-OTC businesses.
Budget carrier Wizz Air reported a 21.9% year-on-year jump in passenger numbers in April, with the Hungarian airline carrying 6.63m people last month.
Wizz Air also said capacity had risen 23.1% to 7.46m seats, while April's load factor came to 88.9% - down 0.9 percentage points year-on-year.