England arrived in Germany as the bookmakers' leading candidates to lift the trophy, but they have failed to live up to the hype in their first two games.
A nervous 1-0 win against Serbia was followed by a drab 1-1 draw against Denmark, sparking scathing criticism from former England stars Alan Shearer and Gary Lineker.
But Kek refused to write off England when it was suggested Slovenia might be able to take advantage of their opponents' unexpected frailties.
"We are playing against one of the favourites, if not the favourite, for the tournament," he told reporters on Monday.
"We respect England very much. We know their strengths and their individuals.
"The Euros have just started and I see them as one of the favourites to win the tournament.
"I don't know why I would question England, be it the individuals or the team in general."
Heading in the last round of group games, England sit top on four points, with third-placed Slovenia and second-placed Denmark both on two points and fourth-placed Serbia on one.
A shock defeat against Slovenia in Cologne on Tuesday would shatter England's hopes of winning the group and earning a potentially easier draw in the last 16.
Slovenia have drawn their opening two games but would progress to the knockout stages if they beat England.
Slovenia and Denmark, who play Serbia in their last game, are currently level on goal difference and goals scored in the race for second place.
Kek said Slovenia would balance their desire to subdue England's stars with the knowledge that a surprise win would send them through.
"Tomorrow's match will be the 'final' in this group. We are still masters of our destiny," he said.
"We will try to neutralise England but we also have the quality to score a goal.
"I know everyone is trying to follow other matches too. Of course it is only human but I am not a fan of calculations. We still have everything in our hands."
Kek was in his first spell as Slovenia boss at the 2010 World Cup when they faced England in their last group game, losing 1-0 as the Three Lions went through at their expense.
Now six years into his second stint in charge of the Slovenians, Kek believes the time is right for his country to avenge that painful loss.
"That was a long time ago. We remember England had great players like (John) Terry and (Frank) Lampard. But now they have many big individuals as well," he said.
"It doesn't matter who we play, England or Malta or anyone else. You have to be brave. This generation of the Slovenian national team deserves something."
Slovenia's Leipzig forward Benjamin Sesko is expected to be fit after suffering a quad injury against Serbia on Thursday.
