* Tunisian says he lost contract after refusing to pay bribe
* SNC-Lavalin already under police investigation in Canada
* SNC CEO resigned in March over improper payments
* SNC stock down around 30 pct in past three months (Recasts with stock fall; adds background)
TORONTO, April 23 (Reuters) - Shares in SNC-Lavalin Group Inc fell more than 3 percent on Monday as new allegations surfaced about impropriety in the North African operations of the big Canadian engineering and construction company.
A Tunisian businessman said he was shut out of SNC's biggest Tunisian construction project because he refused to pay a bribe to one of the Montreal-based company's top North African executives, according to a newspaper report.
The allegations were made by Wajdi Chortani, the general director of Enco Group, a Tunisian construction and engineering company, the report in the Globe and Mail said.
A spokeswoman for SNC could not be reached by Reuters. However in a response to the Globe, SNC spokeswoman Leslie Quinton said the company was not prepared to debate Chortani's allegations in public.
The accusations come at a bad time for SNC, which is already under police investigation in Canada over possibly improper payments and over bribery allegations in Bangladesh. Earlier this month, SNC's headquarters were raided by police in connection with the payments.
On March 26, Chief Executive Pierre Duhaime resigned after an internal investigation found he had broken company rules by authorizing US$56 million in payments to unknown "agents" that were assigned to construction projects that did not exist.
The head of the 100-year-old company's construction operations also left the company in February. SNC has said the improper payments were made at his request.
SNC has declined to say in which country the payments were made but a company statement in March said that US$22.5 million of them were made through a Tunisian subsidiary of SNC.
SNC has also garnered much negative press in Canada for its close ties to the regime of deposed Libyan dictator Muammar Gaddafi.
SNC's shares were down C$1.38, or 3.5 percent, at C$37.86 on the Toronto Stock Exchange on Monday morning. The stock has lost around 30 percent of its value in the past three months.
$1=$0.995 Canadian Reporting By Toronto Newsroom; Editing by Gerald E. McCormick; and Peter Galloway