MONTREAL -- Shares of SNC-Lavalin Inc. plummeted to the lowest level in nearly 15 years Tuesday in reaction to its largest shareholder, the Caisse de depot et placement du Quebec, warning that the embattled engineering firm had to move to emergency mode to improve its project execution.

The Montreal-based company's shares fell to a low of $16.10 in early trading on the Toronto Stock Exchange and were down eight per cent at $16.38 around midday. The TSX was closed Monday because of the Civic Holiday in Ontario and several other provinces.

Caisse CEO Michael Sabia shone the spotlight on SNC-Lavalin Monday during a discussion about the Quebec pension fund manager's results for the first half of 2019. It posted a modest return of 6.1 per cent, well below that of 7.5 per cent of its reference portfolio. Nevertheless, its annualized return of 8.3 per cent over five years exceeds the 7.2 per cent return for the same reference.

The Caisse booked a $700-million loss from its SNC investment during the first six months of the year and Sabia's impatience was clearly evident.

Although Sabia has said a few times that the Caisse is and will remain "a long-term investor in SNC-Lavalin during this turbulent period," he said that the engineering giant "must move quickly and must focus on execution."

Unwilling to comment on the risk of a hostile takeover bid of SNC by foreign investors, Sabia acknowledged that the Caisse remains watchful and that SNC is important to Quebec and to Canada and "the engineering ecosystem in Canada."

Paraphrasing famed inventor Thomas Edison, Sabia said: "A plan without execution is a hallucination."

"That's why we insist so much on the execution, on the daily discipline (...) It is a change of culture, a higher level -- significantly higher -- of discipline."

The Caisse's assets increased $18 billion in the last six months to $326.7 billion.

Sabia said that markets have been less responsive to the economic reality over several months as they have been more concerned about U.S. Federal Reserve action on interest rate cuts than about the real economy.

The Caisse's results were weighed down by sluggish performance in the real estate and infrastructure sectors.

Sabia defended the pension fund manager's "green shift" that aims to increase carbon-neutral investments by 80 per cent between 2017 and 2020 and lower its carbon footprint by 25 per cent by 2025.

Despite purchasing a gas transmission network in Brazil, he said the Caisse is gradually reducing its investments in the oil sector.

"That being said, we believe that gas is a vital source of transition energy," he said.

"We are now probably the most important institutional investor in North America to make investments in renewable energy -- solar, wind turbines -- around the world ... but the world is not going to change instantly and gas is a very important source of transition energy. That's why, very selectively, we make investments in the gas sector."