Tory campaigners cannot sit out the Euro-elections and hand “extremists from the left and right” power, Amber Rudd has said.
The work and pensions secretary said her party faced “the fight of our lives” in the May 23 poll, but that grassroots activists must show “courage”.
It comes as Nigel Farage’s Brexit Party - which is simply advocating a no-deal exit from the EU and no other policy - is predicted to emerge victorious.
Speaking in central London at an event with new Tory think tank Onward, Rudd - widely tipped as a potential successor to Theresa May - urged the party to build a “Conservative Momentum” to rival Labour’s grassroots campaign.
“I want to point out to us all that I believe that we are in the fight of our lives,” she said.
She added: “Extremism is coming up to us from the left and from the right and we need to make sure that we fill the middle.”
The Conservatives will not publish a manifesto for the elections to the European Parliament, with May continuing talks with Jeremy Corbyn to reach a Brexit deal, and the party is said to be haemorrhaging support of both Leavers and Remainers.
But Rudd, who campaigned for Remain and comes from the centre right tradition of the party, said downbeat Tory activists must push “Conservative values” on the doorstep.
“If we leave that vacuum there will be consequences,” she said. “We already have a situation where there is an established politics of grievance, of outrage and what we experience as MPs is that this line of betrayal, of traitors, can lead very quickly to outrage to abuse and can lead to violence.”
Activists must put aside misgivings about May’s leadership and Brexit talks, she told the gathering, which was attended by cabinet ministers and a string of high-profile MPs.
“It’s so easy to keep your head down on this but it is wrong to do so and the best example of that is the Euro elections which are coming up,” she said.
“We have to go out and campaign for Conservative values and Conservative beliefs. Because if we don’t, if we leave that vacuum, the extremist parties will continue to fill it and again that has consequences.”
She added: “It does take a certain courage. But as my great political hero Millicent Fawcett said ‘courage calls to courage everywhere and it will not be denied’.”
Her call follows anti-racism charity Hope Not Hate sounding the alarm over low turnout in the North West offering far-right activist Tommy Robinson a potential MEP slot.
The organisation told HuffPost UK, the independent candidate - whose real name is Stephen Yaxley-Lennon - has a “rock solid” 125,000 votes as it urged voters to back a mainstream party rather than stay at home.