Mr Grant, a political appointee to the Trump administration, advised the Government on how to “switch off” parts of the Brexit Northern Ireland Protocol.
Mr Grant, who has not responded to a request for comment, is a self-proclaimed expert in “sovereignty” and “state immunity”.
In June 2019 he co-authored a paper with Brexiteer Martin Howe QC arguing a no deal Brexit would be a better solution to the negotiations than a deal that would have left Northern Ireland shackled to EU rules, according to the Financial Times.
It comes as a team of US Congress members will travel to London and other capitals as concerns grow in Washington about the tensions caused by Britain’s post-Brexit stand-off with the European Union over Northern Ireland.
At least half a dozen representatives from Congress will hold meetings in Brussels, Dublin, London and Belfast within days.
The delegation will be headed by Richard Neal who chairs the Ways and Means Committee which has powers over trade deals, the newspaper said.
A US House of Representatives aide confirmed details contained in the Guardian story.
Last year US House Speaker Nancy Pelosi said there could be no trade deal between the United States and Britain if the Northern Ireland peace agreement was destroyed.
Senior British politicians want to overhaul the agreement on trade between Northern Ireland and the United Kingdom that they signed up to in order to get Brexit done.
They have warned they might have to take unilateral action.
European Commission vice-president Maros Sefcovic said on Thursday that the possibility of Britain acting unilaterally was “of serious concern” and any move to scrap the agreement would be “unacceptable”.
Boris Johnson is set to visit Northern Ireland on Monday amid a political crisis caused by the DUP blocking the election of a Speaker at Stormont, Sinn Fein vice president Michelle O’Neill has said.
The DUP leader, Sir Jeffrey Donaldson, has insisted he is sending a “clear message” to the EU and the UK Government about resolving issues with the Northern Ireland Protocol.
But Ms O’Neill accused the unionist party of “punishing the electorate” while Alliance Party leader Naomi Long said it had been a “shameful day” for the DUP.
The failure to elect a Speaker leaves the Stormont Assembly unable to function.
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The 90 MLAs met for the first time in the Stormont chamber on Friday after last week’s Northern Ireland Assembly election saw Sinn Fein emerge as the largest party for the first time.
The first order of business was for MLAs to sign the roll of membership before an attempt was made to elect a Speaker. Two candidates, Mike Nesbitt of the UUP, and Patsy McGlone, of the SDLP, were nominated but did not receive the necessary support.
The DUP is also refusing to nominate for the position of deputy first minister, which prevents the forming of a new executive, as part of its protest against the protocol.
Unionists oppose the post-Brexit treaty because of the economic barriers it creates between Northern Ireland and the rest of the UK.