By Sam Tobin LONDON, June 4 (Reuters) - British gambling company Betfair is fighting a landmark London lawsuit that could determine whether betting companies owe a duty of care to problem gamblers, in a case that opened on Thursday and is being closely watched by the industry. The family of Luke Ashton, who died by suicide in April 2021, allege Betfair encouraged him to place bets and that this exacerbated his gambling disorder, and caused or materially contributed to his death.