On April 23 phone users will get a message warning of a pretend life-threatening event.

The exact timing had not been announced — but is revealed in a memo sent to local government leaders and leaked to The Sun.

Tens of thousands will be in the capital for the marathon plus St George’s Day celebrations.

And the cup semi-final between Brighton and Manchester United starts at Wembley at 4.30pm.

A source said: “It’s bizarre timing.

"Many football fans will be drinking. Some runners will be completing the marathon.

"The message could cause panic and havoc.”

The memo says: “We have been informed that the national test message is scheduled for 15.00 on Sunday 23 April.

“The Football Association and organisers of the Marathon have been consulted on this timing.”

The alert aims to warn about a range of threats from wildfires to severe flooding.

Cabinet Office boss Oliver Dowden has said: “It will revolutionise our ability to warn and inform people who are in immediate danger.”

Oliver Dowden has said of the alert: 't will revolutionise our ability to warn and inform people who are in immediate danger'