"Official figures in for England’s opener v Tunisia last night… An 18.3 million peak on [UK TV channel] BBC1 plus another 3 million on the BBC iPlayer. The most-watched TV event of the year… 70% of people watching TV were watching," Walker posted on Twitter.
On Monday, the Three Lions recorded a narrow 2-1 victory in their first match at the tournament against Tunisia with captain Harry Kane netting a header in stoppage time.
Belgium outclassed Panama 3-0 in the other Group G match on Monday, to usurp the top spot in the group.
The 2018 FIFA World Cup kicked off on June 14 and will continue until July 15, with the matches set to be played in 11 cities across Russia.