The bitter rivalry that explains chaos of Starmer's first 100 days

It is the story of a power struggle with a power couple, and it helps to explain the dysfunction at the heart of Sir Keir Starmer’s tumultuous first 100 days in power.

Insiders say that one of the main reasons for the rift between former chief of staff Sue Gray and Morgan McSweeney – who took Ms Gray’s job last week after she was effectively sacked by the Prime Minister – was his marriage to Labour MP Imogen Walker, a fast-rising Parliamentary aide to Chancellor Rachel Reeves.

The sources claim Ms Gray regarded the marital link as a threat to her powerbase. Along with the Prime Minister and Pat McFadden, the Chancellor of the Duchy of Lancaster, Ms Reeves is part of the ministerial ‘Holy Trinity’ which takes key Government decisions.

One Government adviser claims to have heard Ms Gray – within earshot of power-couple members – saying: ‘I do not like political couples.’

The adviser said: ‘It was clearly a dig at Morgan, which aggravated his friends in No 10. She seems less exercised by the issue of plum jobs going to political progeny.’

Ms Gray’s son, Liam Conlon, is a new-intake Labour MP who has won an instant job as a ministerial aide.

Ms Gray was finally forced to resign last weekend after months of negative coverage about her handling of No 10, which snowballed after The Mail on Sunday revealed that Sir Keir was being denied vital security briefings because she was preventing the security services from gaining access to him.

Senior Whitehall sources accused Ms Gray of ‘thinking she runs the country’, suggesting that even Cabinet Secretary Simon Case has been forced to ask her permission to speak to the Prime Minister.

Amid endless rows about freebies and ‘cronyism’, and after it was revealed she had taken a higher salary than Sir Keir’s – at a time when as she was cutting the pay of other advisers – her eventual departure became assured.

No 10 consistently denied that tensions between Ms Gray and Mr McSweeney were destabilising the Downing Street operation in the run up to her ousting.

Ms Walker won her job as parliamentary private secretary to Ms Reeves after being parachuted in as the MP for Hamilton and Clyde Valley. She and Mr McSweeney, who have a young son together, keep their main home in rural Lanarkshire.

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During her tenure in Downing Street, Ms Gray was forced to deny she had scheduled the most important No 10 meetings for Fridays, when McSweeney usually leaves London for Scotland.

The couple met when he was Labour’s campaign manager in Lambeth two decades ago, and turned up at Ms Walker’s house with leaflets about traffic-calming measures in south London. Love blossomed after the Irish-born strategist invited her to join him in singing U2’s With Or Without You during local karaoke sessions.

The McSweeneys are far from alone in having formed a Labour power-axis.

Ms Reeves’s husband is Nicholas Joicey, a high-flying civil servant who is currently second permanent secretary at the Environment Department, while Mr McFadden, the powerful Chancellor of the Duchy of Lancaster and fellow member of the Holy Trinity, is married to Marianna, the former head of insight at the Tony Blair Institute for Global Change, which exerts a strong and growing influence over Labour’s policy agenda.

Deputy Prime Minister Angela Rayner – an ally of Ms Gray who is not involved in a high-profile power union – has been elbowed out of what had been expected to be a ‘Quad’ of decision-makers, and pointedly denied a place on the National Security Council.

The best-known power couple in the Government are Home Secretary Yvette Cooper and Ed Balls, the former Labour Cabinet Minister who now works as a podcaster and breakfast television presenter.

Ms Cooper, who is embroiled in a row over free tickets to a Taylor Swift concert and a police entourage for the singer, is also nurturing the next generation of Westminster coupling: Jess Leigh, one of her key advisers at the Home Office, is in a long-term relationship with Stuart Ingham, a former academic, who works as the Prime Minister’s executive director of policy.

Ms Gray – who continues to attract negative attention over her new ‘non job’ as a regional envoy, her potential severance pay and a widely expected move to the Lords – has adopted a different approach in her private life.

In March 1985 she married Bill Conlon, a Northern Irish part-

time country and western singer from the fishing village of Portaferry in County Down, who has been described as her first and only love.

They met while Ms Gray was taking a mysterious ‘career break’ from Westminster. A church wedding was out of the question as Bill, a joiner, was a divorcee.

The couple took over a pub called the Cove in the middle of IRA ‘bandit country’ on the border with the Irish Republic: she pulled pints while Mr Conlon, the lead singer of a band called Emerald, was in charge of entertainment.

The sojourn, which ended in 1987 when they returned to London, has led to strongly denied claims Ms Gray was working as an agent for British intelligence.