If the incident on Saturday night is confirmed as terrorism, it would mean the UK has faced three attacks in the space of less than three months following the Westminster and Manchester atrocities in March and May.
As the emergency unfolded, Scotland Yard issued a message urging members of the public in the affected areas to “run, hide and tell”.
The guidance was prepared by counter-terrorism police to set out the steps people should follow in the event of an attack.
After the bombing in Manchester, the official terror threat level was raised to the highest – critical – amid fears further attacks could be imminent.
It was the first time the assessment had reached the most serious rating for nearly a decade and a huge security response was activated, with military personnel drafted in to support police.
Last weekend the threat level was lowered to the second highest category of severe, meaning an attack was judged “highly likely”.
It was revealed following the Manchester blast that security services had foiled 18 plots in Britain since 2013, including five since the Westminster attack.
The threat being confronted by the UK’s counter-terrorism agencies is seen as unprecedented, with 500 active investigations involving 3,000 subjects of interest at any one time.
The “terrorist incidents” at London Bridge and Borough Market follow less than a fortnight after the terror bombing at the Ariana Grande concert in Manchester which killed 22 people – the worst terror attack in Britain for a decade.
Here is a timeline of recent attacks:
:: May 22 2017: Twenty-two people – including children – are killed and around 59 injured during a suspected terrorist bombing at a pop concert in Manchester.
Lone suicide bomber Salman Abedi detonated an improvised explosive device as crowds of music fans, many of them youngsters, left Manchester Arena following a performance by US artist Ariana Grande.
:: March 22 2017: Five people are killed when an Islamist extremist launched a car and knife attack in central London.
Khalid Masood drove a hire car over Westminster Bridge, near the Houses of Parliament, mounted the pavement and hit pedestrians before crashing into railings outside the Palace of Westminster.
He stabbed Pc Keith Palmer, 48, to death and also killed US tourist Kurt Cochran, Romanian tourist Andreea Cristea, 31, and Britons Aysha Frade, 44, and 75-year-old Leslie Rhodes. Masood was shot dead by police.
:: June 16 2016: Labour MP Jo Cox is murdered outside her constituency office in Batley, West Yorkshire.
The mother of two, 41, was shot and stabbed multiple times by right-wing extremist Thomas Mair. He was later handed a whole-life prison sentence for her murder.
:: December 5 2015: A lone attacker attempts to behead a passenger during an Islamic State-inspired rampage in the ticket hall of Leytonstone Underground station in east London.
Somali-born Muhiddin Mire targeted strangers at random during the attack before slashing fellow passenger Lyle Zimmerman, 56, with a knife.
The schizophrenic was sent to Broadmoor Hospital after being given a life sentence with a minimum term of eight years for attempted murder.
:: May 22 2013: Fusilier Lee Rigby is murdered by Islamic extremists Michael Adebolajo and Michael Adebowale.
The 25-year-old serviceman was walking near his barracks in Woolwich, south-east London, when the pair rammed him with a car before attempting to hack off his head with knives.
The killers were jailed for life at the Old Bailey in February 2014.
:: June 30 2007: Two men inspired by Islamist extremism ram a 4×4 laden with petrol and propane tanks into the main terminal of Glasgow Airport. One of the attackers died in the incident and five people were injured.
:: July 7 2005: Four suicide bombers kill 52 and injure hundreds of others in blasts on the London Underground network and a bus.