British lawmaker killed in "charged" atmosphere before vote on EU

* Murdered British lawmaker supported "Remain" campaign

* Attacker shouted "Britain first", witnesses say

* Britain First denies links to murder suspect

* Anti-racism activist speaks of febrile campaign atmosphere

By Michael Holden

LONDON, June 17 (Reuters) - In the weeks before lawmaker Jo Cox was killed, there were warnings that passions could spill into violence in the fevered campaign for next week's referendum on Britain's membership of the European Union.

Police have declined to comment on the motive for Thursday's murder, but the attack has raised questions about whether those fears have now been realised.

Cox, a supporter of the campaign to stay in the EU, was shot and stabbed by a man who witnesses said shouted "Britain first" -- a rallying cry for some supporters of the "Leave" campaign but also the name of a right-wing group.

Leading figures from the "Remain" campaign have made no link between Cox's death and the referendum campaign, which has become increasingly angry and bitter in the latter stages.

But on social media some Britons highlighted a warning last month by Nigel Farage, leader of the anti-EU UKIP party and one of the most prominent figures in the referendum campaign.

"I think it is legitimate to say that if people feel they've lost control completely, and we have lost control of our borders completely as members of the European Union, and if people feel that voting doesn't change anything then violence is the next step," Farage told BBC TV in last month's comments.

He went on to say he found it "difficult to contemplate it happening here but nothing is impossible", and said after the attack that he was horrified and deeply saddened.

One of the most emotive issues during the campaign for the June 23 referendum has been immigration.

Supporters of a British exit, or Brexit, say that uncontrolled immigration from the rest of the EU has put pressure on jobs and services, and that millions of Turks could arrive in Britain if Turkey joins the 28-country bloc.

"Taking place inside the febrile and increasingly-charged atmosphere of the EU referendum debate -- with the swing to anti-immigrant sentiment -- for some that may swing them towards hate, and for a smaller minority perhaps even violence," Nick Ryan, of the anti-racism group Hope Not Hate, told Reuters.

Tempers have also flared over accusations of scaremongering by both sides and allegations by each that the other is distorting the facts, especially on the cost of EU membership to Britain and the likely economic impact of a Brexit.

The "Remain" campaign led by Prime Minister David Cameron says an exit would be a "leap into the dark" for the country and its economy. The "Leave" campaign says a Brexit would liberate Britain to trade more freely with the world.

FAR-RIGHT LINKS?

British media have named the suspected killer arrested by police as 52-year-old Thomas Mair, whose brother said he had a history of mental illness and no strong political views.

But a U.S (Other OTC: UBGXF - news) . civil rights group said a man by the name of Thomas Mair had been associated with a neo-Nazi organisation since 1999, and the cry of "Britain first" heard by witnesses raised the possibility the attack was politically-motivated.

Among those to use the phrase is a small Christian, right-wing nationalist political party called Britain First whose motto is "Taking Our Country Back".

The party, set up in 2011, wants to halt immigration, deport all illegal immigrants and make it an act of treason to transfer any sovereignty to a foreign institution.

But its leader, Paul Golding, has distanced the group from any link to Cox's killing, which he called a "despicable crime", and said it had no connections with Mair.

"What this person said -- was he referring to an organisation, was he referring to a slogan, was he shouting out in the middle of an EU debate 'It (Other OTC: ITGL - news) 's time we put Britain first'?" he said in a video statement on the group's website.

"I've heard this almost every day. It's the name of our party yes ... (but) everyone is saying it's time we put Britain first, it's the type of language that's been utilised during this referendum campaign."

The party has 1.4 million "likes" on Facebook (NasdaqGS: FB - news) and anti-fascist campaigners say its ranks include former members of the British nationalist Party (BNP (Paris: FR0000131104 - news) ), a far-right party that won two seats in elections to the European Parliament in 2009.

But Britain First remains a fringe party, having mustered just 1 percent of first-round votes when Golding stood in an election for London Mayor in May.

Golding turned his back when it was announced that Labour's Muslim candidate, Sadiq Khan, had won the election and although the party rejects accusations it is racist, it says it wants to introduce a comprehensive ban on Islam.

It also recently held its first activist training camp in the mountains of north Wales, with a film on its website showing members wearing military fatigues holding Union flags, boxing and practicing self-defence.

Support for far-right parties in Europe has grown as the continent struggles with an influx of migrants, most notably in Austria where far-right candidate Norbert Hofer almost won a presidential election in May.

But Britain has not seen any notable rise in far-right public backing, and the BNP has faded into obscurity. (Editing by Guy Faulconbridge and Timothy Heritage)