The FA Cup Manchester derby at Wembley will reportedly be the most expensive security operation ever for a single game in England.
The draw has led to calls for Manchester City and United to be played anywhere but Wembley - with some journalists and fans suggesting that the game be moved to Villa Park.
But cup semi-finals have been held at Wembley since 2008 in order to recoup some of the cost of building the national stadium.
Over 80,000 fans will need to be policed - with the majority travelling down from Manchester on the weekend of April 16-17.
If City reach the Europa League quarter-finals, the game is likely to be moved to the Sunday - which would clash with both the London Marathon and a Premier League clash between Arsenal and Liverpool.
The massive operation will involve Scotland Yard, Greater Manchester Police and British Transport Police, who will need to keep supporters apart on trains between Manchester and London.
A Metropolitan Police spokesman said: "We deal with football matches - FA Cup semi-finals and final, internationals - all the time.
"We obviously have a lot of experience of dealing with these things and an appropriate and flexible policing policy will be in place."
A Scotland Yard source had earlier told the Daily Mail: "From a security point of view, this is a complete nightmare. It will cost millions of pounds."