Morgan McSweeney has near-mythical status in Westminster. More than anyone, this Irishman from County Cork is credited with wresting control of the party from the Corbynite hard Left, ensuring Sir Keir Starmer won the leadership and delivering the 2024 landslide. Having built the House of Starmer, the 49-year-old could now bring it crashing down.

If there is the slightest divergence between his testimony about the appointment of Peter Mandelson as US ambassador and what Sir Keir has told MPs then the Prime Minister will be accused of misleading Parliament and his foes will pounce. They will seize on his utterances today and urge MPs from all parties to vote to refer the PM to the privileges committee. Anything which bolsters their argument that "due process" was not followed at all times will be weaponised.

Even if he does not outright contradict Sir Keir's statements, he can hasten the end of the Starmer era. The Prime Minister is in a position of grave weakness and there is the damaging perception that he does not have a grip on Government, that aides and officials have pushed through decisions which have proved catastrophic for Labour.

If Mr McSweeney deepens the worry in Labour circles that the Government is run by a passive PM who cannot grasp the political danger of appointing Mandelson to the top diplomatic post, of slashing entitlement to winter fuel payments or hiking inheritance tax on farmers, more MPs will decide he has to go.

Opposition parties are hollering for Sir Keir to quit over his decision to send Mandelson to Washington DC but it is Mr McSweeney's head that has rolled. It would not be surprising if the former Downing Street Chief of Staff feels badly bruised but he is unlikely to be out for revenge. As with Mandelson, he has spent the best years of his adult life striving to make Labour electable, and he won his spurs by helping stamp out support for the BNP. He will not want to burn down the most successful vehicle for centre-Left politics in British politics.

But when the next batch of communications between Mandelson and Government figures is released there is every chance Mr McSweeney will be called back for more questioning.

MPs - and citizens across Britain - want to know why Downing Street was enthralled by Mandelson and about his influence on wider decision-making within the Government. Were they frightened not to give him the job, or did they feel they could not do their jobs without him in post?

There is a sad irony for Mr McSweeney as he surveys the turmoil which has gripped his party. The chaos in Government and the disillusionment in the country has fuelled the restoration of the hard Left as a major force. This time the Left-wingers who have captured the imagination of change-hungry Britons are not Labour MPs but Green activists who threaten to oust swathes of councillors in the May 7 elections.

In 2024, the New Statesman put Mr McSweeney in first place in its list of the most powerful people in Britain - one position above Sir Keir. Mr McSweeney is out of Government but he still wields tremendous power, and what he says today and in the coming weeks could trigger the arrival of removal vans at Downing Street.