LONDON - Three of six opinion polls published ahead of a June 23 referendum showed a shift towards keeping Britain in the European Union, but the race still looked too close to call.

Britain's Prime Minister David Cameron drinks from a pro-Remain mug during a meeting with Jeremy Clarkson and James May in a studio in west London, June 16, 2016. [Photo/Agencies]

The two polls based on the freshest surveys of voters - which were conducted almost entirely after the killing on Thursday of a British lawmaker - both put the "In" camp in the lead, reversing a recent rise in support for "Out".

Prior to Saturday's polls, in the period between June 10 and June 16, "Out" led in seven of nine polls, rattling investors.

Betting odds, provided by gambling firm Betfair on Sunday, gave the "In" campaign a 69 percent probability of winning, up from as low as 59 percent on Thursday.