The Metropolitan Police commissioner Sir Ian Blair has warned Londoners to be on their guard for possible terrorist attacks in the near future.
Sir Ian Blair warned of the threat of possible attacks
Addressing Thursday's meeting of the Metropolitan Police Authority Sir Ian said it was a critical time.
A forthcoming election, royal wedding, G8 meetings and Britain's EU presidency all added to terrorism risks, he said.
The warning comes as workers at Heathrow's Terminal 5 were told they would be subject to background checks.
Sir Ian said it would be "unwise" for him to speculate on whether there had been any specific threats, but drew parallels to last year's bomb attack on a Madrid train station which killed 170 people days before the Spanish general election.
He said: "Terrorists have long memories. They understand what happened in Madrid and know what the impact of that was on the Spanish electorate.
"This year we are responsible for the EU presidency, presidency of the G8, a royal wedding and a general election. There are obvious and enormous targets which we have to deal with."
This week, an email was sent to staff by Heathrow Terminal 5's head of security saying workers' details would be given to police for "counter-terrorist checks only".
Terminal 5 is due to be completed in three years' time
Terminal 5, one of the biggest building projects in Europe, had previously only run standard employment checks.
Construction on the terminal, which is due to be completed in three years time, is more than 50% complete.
Terminal 5 workers threatened to strike last month because of claims security checks were helping to add 90 minutes a day to their journey times.