LONDON (Reuters) - Britain s Electoral Commission said on Thursday it was investigating campaign group Momentum - which backed the opposition Labour Party at a June election - over whether it breached campaign finance rules. Momentum is closely associated with left-wing Labour leader Jeremy Corbyn and has helped the party attract new voters, contributing to its unexpectedly strong performance at the June vote in which Conservative Prime Minister Theresa May lost her parliamentary majority.  The Electoral Commission has today announced it has opened an investigation to establish whether Momentum, a registered non-party campaigner at the 2017 UK Parliamentary General Election, breached campaign finance rules in relation to spending,  the commission said in a statement. The Electoral Commission, Britain s election watchdog, said it was looking into a range of issues, including whether the campaign group exceeded spending limits or submitted incomplete or inaccurate expenditure records. Momentum is registered as a  non-party campaigner  and is subject to different spending rules to Labour and other political parties.  Momentum are a high profile active campaigning body. Questions over their compliance with the campaign finance rules at June s general election risks causing harm to voters  confidence in elections,  said Bob Posner, the Electoral Commission s Director of Political Finance and Regulation and Legal Counsel. 