Stephen Flynn is set to warn that Westminster parties have "turned the UK constitutional debate into a full blown crisis of democracy".
The SNP Westminster leader will appeal to Labour and the Tories to "free themselves from the dead end of denying democracy" in his first speech outside of parliament since he replaced Ian Blackford last month.
He will also warn that a "just say no to democracy" strategy is doomed to fail and will see support for independence continue to rise in Scotland.
He is expected to say: "At the Scottish Parliament election, a record majority of MSPs were elected on a mandate to hold an independence referendum. "In a functioning democracy, the UK government would have accepted that democratic vote by the people of Scotland and agreed to a referendum.
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