The prime minister said he was "staggered" to find out last week that civil servants in the Foreign Office withheld information from him.
The prime minister says he only learned of security concerns around the ex-US ambassador earlier this week.
British Jews have described to BBC Panorama how they are experiencing a rise in antisemitism.
Elections are taking place across the country in May. Use our postcode lookup to find out if there are elections where you live and who you can vote for.
The incident took place in central London in the early hours, leaving a woman in a critical condition.