The Alba party leader claimed that “desperate unionists” were seeking to drag the monarchy into the constitutional debate in the wake of the re-election of an SNP Government earlier this month.
He also questioned the actions of Prince William saying the meeting with former PM Brown, who has recently rebranded his think-tank ‘Our Scottish Future’ as a pro-union campaign organisation, was unwise.
Kensington Palace confirmed after the meeting the Prince had been listening to views on independence from different communities.
However, in a video address to supporters, the former first minister said it would be a "fatal error” for the Royals to “take sides”.
“Whatever Prince William thought he was doing by agreeing to a confab with Gordon Brown just as he set up his latest campaign for the union … we should have no hesitation in seeing from that meeting, unionist preparation for attempting to persuade a constitutional monarchy to intervene in a constitutional debate which should be left to the people,” said Mr Salmond.
“It would be very wise for the royal family to follow what has been the Queen’s example over her long reign, to keep the monarchy over and above politics, and it would be foolish to have a situation in which it can be said that the monarchy is involved in the Scottish constitutional debate.”
He added: “Back in 2014 David Cameron made a last ditch desperate and failed attempt to drag the Queen into politics. The Brown meeting shows poor judgement on both sides, but the same degree of unscrupulous unionist desperation. The independence team should take note."
Mr Salmond said despite his party’s poor showing at the election membership was growing, and the independence case “must be progressed as a matter of urgency”.
He added: “It is quite clear that the forces of the union have no compunction in getting themselves ready and set for what is coming in the next referendum campaign.
"However, the Scottish Parliament elections have left them in a state of disarray. That is why the case for Scottish independence must be progressed now as a matter of urgency.”