Amid heightened fears of a terror attack, security is being stepped up for the start of the Wimbledon tennis tournament today in England.

Scotland Yard said changes have been made to policing plans for this year’s tournament, and the UK’s terror threat level is at “severe,” meaning that an attack is “highly likely,” The Telegraph reports. Extra officers will be on patrol in uniform and plainclothes, and a “highly mobile” reserve unit is ready to respond to any incidents, the law enforcement organization said.

The shift in policing comes after at least 15 British tourists were killed in the terrorist attack on a Tunisian beach on Friday. Next week will also mark the 10th anniversary of the July 7 bombings