The prospect of Donald Trump making a state visit to Britain in June is stomach-churning. The corruption investigation led by special counsel Robert Mueller, whose damning report was published last week, provided ample evidence of what we already know: Trump is unfit to hold the office of president of the United States. By his words and actions over two wretched, destructive years in power, he has proved beyond doubt he is no friend of Britain.

The proposal that the British state should extend to this unworthy man its highest honours, including an address to parliament, and a banquet and carriage ride down the Mall with the Queen, is misjudged. It will do nothing to revive the “special relationship”, already torn apart by Trump’s reactionary policies on climate change, migration, race, multilateralism, Yemen, nuclear arms, civil liberties and other issues. What it will do is give an undeserved boost to a wounded charlatan.

This scandal looks certain to escalate, not fade. Barr’s many redactions will not be allowed to stand

If Americans are content to allow a habitual liar who has presided over systemic illegality, numerous ill-concealed attempts to obstruct justice and a foul-mouthed culture of venality and vendettas to continue to lead their country, that is a matter for them. But the British people cannot be expected to collude or condone such misbehaviour. And what’s to be gained? A fantasy post-Brexit trade deal? Trump’s word, evidently, cannot be trusted.

Trump and his supporters are hoping the Mueller fallout will quickly dissipate. Sycophantic attorney-general William Barr’s belated release of the report the day before Easter, like his earlier, misleading contents “summary”, was a dishonourable attempt at damage limitation. Yet this scandal looks certain to escalate, not fade. Barr’s many redactions, particularly concerning the Trump campaign’s involvement with Russian election meddling and the WikiLeaks connection, will not be allowed to stand. Mueller refers to 14 ongoing, related criminal investigations. Separate federal probes continue in New York.

The onus now falls on Congress to take the vast amount of information gathered by Mueller and move the process to a conclusion. Leading Democrats have signalled their intention to do so. Additional testimony will be sought from both Mueller and Barr. Some, like the presidential hopeful Elizabeth Warren, are pushing for impeachment. Others, like the House speaker, Nancy Pelosi, want to further weaken Trump by besieging him with endless, enervating inquiries.

Democratic Senator Elizabeth Warren was the first presidential candidate to call for the US House of Representatives to begin impeachment proceedings against Donald Trump. Photograph: Brendan Smialowski/AFP/Getty Images

Such overtly political calculations are unhelpful. So, too, is criticism of Mueller himself. He performed an intensely difficult task with dignity and discretion. There is no evidence that Trump’s attempts to bully him, and threats to sack him, influenced his findings. According to his report, he ultimately felt unable to bring criminal charges because that would breach justice department rules disallowing prosecution of a sitting president. Amid so much impropriety, this was the properly legal course.

But Mueller has nevertheless lit the fuse to a very large bomb located under Trump’s Oval Office desk. It seems clear that, but for that rule on prosecutions and the refusal of top aides to carry out his illegal orders, Trump would already be in the dock. By identifying 11 instances of possible obstruction of justice, Mueller has effectively laid out a road map, and grounds for future indictments, that Congress – Democrats and Republicans – has a duty to pursue.

What has been alleged, and to some degree proved, is not, as Trump’s defenders claim, the product of partisan attempts to destroy him. It concerns grave wrongdoing at the heart of the presidency. It amounts to the biggest crisis in US governance since Watergate. At stake is Americans’ trust in their democracy and the effectiveness of their vaunted constitutional doctrines. Also in the balance is America’s standing in the world. Until the door shuts on Trumpgate – and the sooner the better – Trump will remain an international liability. He is not welcome here.