Birsa, who scored a superb goal in the 2-2 draw with the United States when the Slovenians threw away a two-goal lead, believes England cannot play any worse than they did in the 1-1 draw with Americans and a goalless stalemate with Algeria.
"It would be foolish to think England are not a top-level team after underperforming twice under heavy pressure from their fans," Birsa told reporters in Slovenia's Hyde Park training base on Monday.
"They came here with the ambition to win the World Cup and going out in the group stage would be a disaster for them so I am sure they will improve beyond recognition for us."
A draw would see the Slovenians through to the second stage while even a defeat would not eliminate them if the outcome of the Algeria v United States game played into their hands, but Birsa said they would rely solely on themselves.
"Our destiny is in our own hands and if we can reproduce the first-half performance against the Americans we have a good chance of getting a result against England," he said.
"There is no point in looking for favours elsewhere because we shouldn't need or expect lucky breaks in matches we are not involved in. We have to try to stifle out England the same way their previous two opponents did and be patient."
