That is absolutely untrue. This is the position now in the Fixed Term Parliament Act:The Queen has no power in a formal sense to do anything with Parliament. It's pure pomp and ceremony.
"The royal prerogative to dissolve Parliament was abrogated by the Fixed-term Parliaments Act 2011. Section 6(1) of the Act however specifically states that the monarch's power to prorogue Parliament is not affected by the Act."
In fact the Queen could even have stopped Brexit by refusing to sign any of its acts. Since the 2017 Queen's Speech specified that eight further acts are necessary to implement all Brexit stages, she has plenty of opportunities to still do so.
Of course it's unlikely she will intervene, but she can.
She will probably be the last monarch to have such sweeping powers since they are granted by parliament at the time of coming to the throne. It's widely believed that Prince Charles will not have all of them granted then.
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