UPDATE 5-Gov't of Canada's British Columbia toppled in non-confidence vote

(Adds comments from province's next premier and province's nominal head)

VICTORIA, British Columbia, June 29 (Reuters) - British Columbia's Liberal government was defeated on Thursday in a non-confidence vote, as expected, paving the way for the left-leaning New Democrats to rule the Western Canadian province for the first time in 16 years.

Such a prospect has unnerved investors in Canada's third-most populous province, not least owners of oil and gas projects such as Kinder Morgan Inc's C$7.4 billion Trans Mountain pipeline expansion, which the New Democratic Party (NDP) has vowed to halt.

But an NDP government, which has to be propped up by the third-place Green Party to achieve a slim parliamentary majority of one, is fragile and few expect it to survive the four-year term.

On Thursday, seven weeks after a knife-edge election, NDP and Green lawmakers used their 44 votes in the 87-member legislature to introduce a non-confidence amendment in the Liberal government's Throne Speech.

After the vote, NDP leader John Horgan told reporters he has met the province's nominal head, Lieutenant-Governor Judith Guichon, and that she has invited him to form a new government, making him British Columbia's next premier.

"We'll have access to government documents tomorrow to start working on a transition," Horgan said. "I can't predict when that (transition) will be, but it's going to be soon."

Incumbent British Columbia Premier Christy Clark, who met Guichon before Horgan, was expected to resign in the meeting, although she did not address the issue with reporters later and declined to take questions.

Guichon said in a statement she "will" accept Clark's resignation.

Green leader Andrew Weaver called the confidence vote a "historic" opportunity.

Weaver's party and the NDP struck an agreement last month to oust the right-leaning Liberals - unaffiliated with the left-leaning Liberals in power federally - after a May 9 general election reduced Clark's party to a minority.

The NDP and Greens, which will form the first minority government in the province in 65 years, have accused the Liberals of trying to cling to power after the election by stealing their election promises and introducing them as last-minute legislation to delay being voted out.

Yet those same promises could be hard to deliver under an NDP government, which needs Green cooperation and every member to be present for every vote to pass laws, said University of British Columbia political science professor Hamish Telford.

"The NDP may decide on its own accord that it needs to have a fresh election," he said. (Reporting by Nicole Mordant in Vancouver, additional reporting by Ethan Lou in Calgary, Alberta; Editing by Sandra Maler and Himani Sarkar)