Mercedes, Ferrari, Renault and Honda between them supply the 10 teams and there are no newcomers set to join them in two years' time.
"The drawbridge has been pulled up and the existing suppliers don't want anyone else to come in," said Brawn, a former Honda and Mercedes team boss who was also previously technical director of Ferrari.
He said a compromise had been found, however, that might make it easier.
"There are regulations coming out which would mean new entrants will get support from existing entrants. There will be components and technology which will have to be shared if it is requested.
"There has been a recognition from the existing manufacturers that they can’t shut the door behind them," said the Briton.
"If we start to get serious interest from another manufacturer or supplier, they have to cooperate to find ways of helping that manufacturer come into F1."
Formula One’s post-2020 vision set out last April originally envisaged cheaper, simpler engines to encourage new manufacturers into the sport, with Porsche and Aston Martin attending some meetings.
Aston Martin chief executive Andy Palmer told Reuters last month that his company had lost interest after the sport backtracked on those plans.
The four existing suppliers have been wary of incurring significant costs in engine development just as the power gaps are closing.
Brawn acknowledged that what was not being proposed "is not quite such a radical change" but still represented "quite a good step" in the right direction.
The Briton also said changes for next season, with cars boasting bigger, wider and simpler front wings, should promote closer racing.
"Until the cars run, we don’t know what solutions they have made, but from predictions, we’re achieving about 20 percent improvement," he said. "So we’re about a quarter of the way there to where we think we could be.
Brawn said Formula One was on course to introduce a budget cap in 2021.
"We are having pretty constructive discussions with the teams," he added. "Unlike previous initiatives, this budget cap is going to be embedded in the regulations of the sport."
(Reporting by Alan Baldwin, editing by John Stonestreet)
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