london britain electoral commission say thursday investigate campaign group momentum back opposition labour party june election whether breach campaign finance rule momentum closely associate left-wing labour leader jeremy corbyn help party attract new voters contribute unexpectedly strong performance june vote conservative prime minister theresa may lose parliamentary majority electoral commission today announce open investigation establish whether momentum register non-party campaigner 2017 uk parliamentary general election breach campaign finance rule relation spend commission say statement electoral commission britain election watchdog say look range issue include whether campaign group exceed spend limit submit incomplete inaccurate expenditure record momentum register non-party campaigner subject different spend rule labour political party momentum high profile active campaign body question compliance campaign finance rule june general election risk cause harm voters confidence elections say bob posner electoral commission director political finance regulation legal counsel uk electoral body say investigate labour-backing campaign group momentum