Queen Elizabeth II's funeral and mourners who attend the event could be a "target for terrorists," ex-Met Police Chief Parm Sandhu said on BBC Radio 4's World At One program.
"London could potentially be full," Sandhu said. "We are expecting to have people queuing for 12, 15 hours at a time just to file past the coffin, which is a real concern because crowds attract criminals."
She said that the crowds gathering for the queen could make London busier than it was for the deaths of Elizabeth's mother or even Princess Diana, and she said it could also be busier than when London hosted the 2012 Olympic Games.
"I don't ever remember London expecting to be full in this way, even with the Olympics," she said. "The crowds themselves will be a target for terrorists. The funeral itself will be a target for terrorists."
"All the services are going to be stretched and with the queues that are going be right across central London, you could have streets that are going to be quite difficult to walk down," she added, noting that getting a car in the city will be particularly difficult as well.
Queues to pay respects for the queen as she lies in state at Westminster have already begun, though they do not officially open to the public until Wednesday evening local time.
The queues will remain open until Monday, September 19, when the Queen Elizabeth II's funeral will be held.