For Leighton shareholders, the final indignity has yet to be delivered.

The besieged construction group, now in the clutches of Spanish outfit ACS after today's ousting of its two most senior executives, has been grappling with the thorny issue of yet further write-downs.

Insiders estimate Leighton could be facing write-downs of more than $2 billion and say that the board has been mulling the question for months.

The bad debts relate to a blow-out in costs for a jetty at the massive Gorgon LNG project in Western Australia, along with unpaid receipts from its controversial Middle East operation Habtoor Leighton.

Write-downs from the Middle East operation alone could amount to almost $900 million, according to some analysts, while US energy giant Chevron late last year announced a further $2.2 billion cost blowout on the Gorgon project.

A major part of that cost overrun relates to Leighton's contract to build the jetty for the project, which has been plagued by scope changes and higher than expected labour costs.

Write-downs are a touchy subject for the once great construction group.

In April 2011, just four months after the departure of Wal King as chief executive and weeks after forecasting a healthy profit, the company wrote off $1.1 billion in future earnings that sent its result into the red, resulting in a shareholder class action.

Most of those losses related to Australian infrastructure projects such as the Brisbane motorway and the Victorian desalination plant, but the Middle East also featured prominently. A year later it backed that up with a further write-down on its Brisbane Airport link project.

While its latest result, delivered last month, was largely better than expected, receivables rose sharply to a massive $5.1 billion.

The company attempted to attribute the delayed payments – mostly relating to "LNG projects" – to lengthy payment cycles and "complex" negotiations around contract variations.

The shock departure this morning of chief executive Hamish Tyrwhitt and chief financial officer Peter Gregg may well have been a clearing of the decks as frustration over the delayed payments, along with an ongoing corruption inquiry by the Australian Federal Police, has boiled over into the boardroom.

The sweetened offer from Spanish group ACS – which now controls Leighton's major shareholder Hochtief – has been pitched at grabbing a 74 per cent stake in Leighton.

The reason for stopping short of a full takeover is that a holding above 75 per cent would trigger a refinancing of Leighton's complex debt structure.

Given the parlous state of Europe's construction industry in recent times, that is something the Spanish would prefer to avoid.

Leighton Holdings timeline 1949 Leighton born The company was started in Melbourne by British immigrant Stanley Leighton 1962 Company floated Listed on the Melbourne Stock Exchange 1983 Hochtief takes stake Germany's largest contractor Hochtief becomes a major shareholder; Leighton Holdings acquires rival contractor Thiess 2000 John Holland deal Acquires a majority stake in another big contractor, John Holland 2007 Middle East Expands into the Middle East buying a 45 per cent stake in Al Habtoor Engineering. Share price peaks at $62.45 in December 2009 GFC crash The global financial crisis, Leighton's debt and a collapsing Middle East market take a toll, and the share price collapses under $16; the company's value falls by 75 per cent in just 2 years 2010 Spanish move Spanish construction giant Grupo ACS acquires a controlling stake of Leighton's majority shareholder Hochtief Jan 2011 King dethroned Veteran CEO Wal King resigns and is replaced by David Stewart Apr 2011 Massive loss $1.1 billion writedown to major projects announced - forecast $480 million profit swings to $427 million loss, the company's first in more than 20 years; cost blowouts at Brisbane Link and Victorian desal Aug 2011 Another CEO exits CEO David Stewart forced out after just 9 months and replaced by Hamish Tyrwhitt Feb 2012 Federal investigation AFP investigation starts into reports of corrupt payments made to win win contracts in Iraq Oct 2013 Scandal revealed Details emerge about the AFP investigation: allegations of a $42 million bribe made by subsidiary Leighton International to win an oil pipeline contract in Iraq Mar 2014 ACS bids for more control Hochtief moves to increase its holding in Leightons fro 59% to 74% with a with a $1.25 billion offer. CEO Hamish Tyrwitt and Chief Financial Officer Peter Gregg resign. ASIC investigates "unusual" share trading that took place before the bid became public

You can see more about Leighton Holdings on The Business tonight at 8:30pm (AEDT) on ABC News 24 and after 11:00pm on ABC 1.