New Democratic Party MLA Gregor Robertson resigned his seat in the British Columbia legislature on Thursday to focus on his campaign to become mayor of Vancouver.

The moves comes just days after the MLA for Vancouver-Fairview won a nomination vote, making him the Vision Vancouver's party's mayoral candidate in civic elections in November.

Since he secured the nomination on Sunday, speculation has swirled about whether Robertson would give up his seat in the B.C. legislature.

"I have taken a couple of days to consider the situation after an amazing nomination vote last Sunday," he said in a statement.

"I have decided that all of my energy must be focused on reaching out to all of the citizens of Vancouver, and gaining their input, insight, and support for my Mayoral bid, and the Vision Vancouver campaign."

His resignation is effective mid-July, which will allow him time to make the transition, he said.

After the announcement, NDP party Leader Carole James said she will hold off on re-shuffling her cabinet through the summer in case there are other MLAs who decide not to seek re-election.

Robertson's resignation will force a by-election in Vancouver-Fairview in the spring of 2009, and party rules dictate the NDP must run a female candidate in that riding.

Robertson has held the seat since 2005, when he narrowly defeated Liberal candidate Virginia Greene.

Since then, the NDP's small business critic has been an advocate for Cambie Street merchants, who say their businesses have been hit by construction on the Canada Line transit system. Robertson has also lobbied for more affordable housing and environmentally-friendly initiatives in B.C.

Earlier this week, Robertson said if he held his seat in the legislature while campaigning for office he would not be the first to do so. The comment was a reference to Premier Gordon Campbell, who campaigned to become leader of the B.C. Liberal party while he held the city's top job in 2001.

The Vancouver born-and-raised entrepreneur worked in organic farming industry and created organic fruit juice company Happy Planet before entering into politics in 2005 as a relative unknown.

Robertson is facing two-term Non-Partisan Association Councillor Peter Ladner in the battle for the mayor's chair in November.