London Stock Exchange boss Xavier Rolet faces a crucial test this week as TMX Group owners vote on their planned merger with the LSE -- a deal on a knife-edge that is likely to define his tenure.

LSE and TMX shareholders will vote simultaneously in London and Toronto on Thursday at 10 a.m. EDT on the $3.6 billion plan to create a transatlantic exchange group and take the LSE into North America for the first time.

The LSE faces stiff opposition from Maple, a 13-strong group of Canadian financial firms, whose $3.7 billion bid trumps the LSE's. Maple has urged TMX shareholders to block the LSE deal.

I suspect the LSE shareholders will approve the deal on June 30, but it is a close call which way the TMX shareholders will go, said James Hamilton, an analyst at Numis Securities.

LSE shareholders contacted by Reuters last week were largely supportive of the deal, attracted by promised cost savings. But one noted it could be tough convincing TMX's investors.

If he loses out, I think it will be because he hasn't been able to target private investors of TMX and because politically it's been such a nightmare, said one of the LSE's largest shareholders speaking anonymously.

TMX shareholders last week said the British exchange must hike its stock-and-cash bid to win their backing.

TMX continues to support its friendly deal with the London bourse, which it struck in February.

The TMX deal is crucial for Rolet, who took over from Clara Furse as LSE CEO two years ago. It is his first serious foray into mergers and acquisitions, an area that makes or breaks many chief executives.

Rolet would be CEO of the enlarged group, which would likely be too big to be a target for rivals such as Nasdaq OMX , which tried and failed to buy the LSE twice in 2006.

Another challenge Rolet faces is convincing Canadian regulators the deal is beneficial for the national economy, a key requirement for them to approve it.

Maple, meanwhile, faces antitrust concerns about a small group of trading firms controlling the national stock exchange.

(Additional reporting by Cecilia Valente, Thomas Wilkes and Christopher Vellacott; Editing by Will Waterman)