Carillion plc  (CIOIF) shares plunged Monday after it hit investors with a shock profit warning, a big write-down to the value of some of its contracts and said its CEO has stepped down.

The London-listed construction and support services firm said that first-half operating profit will be lower than previously expected due to the timing of some asset sales it had been banking on pulling off. It will also write down the value of some of its work contracts by £845 million ($1.09 billion), mostly contracts in the Middle East and the U.K., and has forecast that full year revenue will now be between £4.8 billion to £5 billion.

Carillion shares fell as much as 38% to 113.8 pence, its lowest level since March 2003, marking a 49% loss for the year to date.

The contractor has the most shorted stock on the London Stock Exchange, with more than 23 of the shares out on loan to short sellers according to the latest data from regulator the Financial Conduct Authority.

It bloated balance sheet has been a key source of concern for investors and, given the poorer than expected performance during the period, it will now miss its target for reducing debt.

"Despite making progress against the strategic priorities we set out in our 2016 results announcement in March, average net borrowing has increased above the level we expected, which means that we will no longer be able to meet our target of reducing leverage for the full year," said non-executive chairman Philip Green.

Green has stepped into the hot-seat at Carillion after CEO Richard Howson took the decision to step down in response to the torrid performance.

The revised revenue guidance implies that the company could now see a large portion of the previous year's growth reversed.

In 2016 Carillion saw its top line rise by around 14% to £5.2 billion although it continued to take on new debt at a rate that has left investors concerned.

Total debt came close to £600 million for the period, while the pension deficit was just more than £600 million, which led management to pledge some form of reduction for 2017.

"The shares trade at a significant discount (c.35%) to the sector and are, in our view, pricing in a rights issue to ease the balance sheet difficulties," said Sam Cullen, an analyst at Jefferies, in an earlier note.