BREXIT is a "job half done" and Ukip MPs will be needed to see it through to the end, Paul Nuttall has insisted as he launched his party's election campaign.

At a hotel in central London, the MEP accused Theresa May of "backsliding" on immigration and said a "whopping Conservative majority" would jeopardise Brexit from happening.

The Ukip launch was disrupted for a short time after several members of the Stand Up to Racism group infiltrated the launch, supposedly posing as journalists, and protesting at what they regarded as the party’s racist policies.

The demonstration came after Ukip announced its commitment to ban full-face veils in public, crack down on female genital mutilation and Sharia courts, and block the opening of new Islamic schools.

Arron Banks, the former Ukip donor, accused the party of waging a "war on Muslims"; a claim denied by Mr Nuttall.

He confirmed the party was prepared to stand aside in dozens of constituencies to help sitting MPs, regarded as real Brexiters, hold on to marginal seats.

Accusing Mrs May of "flagrant opportunism" in calling a snap election, the Ukip leader insisted his party would fight it with "vigour".

He told activists: "A whopping Conservative majority will only serve to put Brexit in peril. Hordes of Tory lobby fodder will allow the Prime Minister to backslide safe in the knowledge that she has the votes banked.

"We are not convinced that the Prime Minister, who campaigned to Remain in the referendum, will get the deal the British people want. She is already beginning to backslide on immigration with the Government now telling us that immigration will run at today's level for the next decade.

"She has said nothing to guarantee our waters and protect our fishermen and she will not rule out paying a huge divorce bill once we have left the European Union," declared Mr Nuttall.

Despite widespread expectation that he will fight the strongly pro-Brexit constituency of Boston and Skegness on June 8, the party leader declined to confirm his plans but promised to reveal them on Saturday.

"Anyone hoping that Ukip is going to fade away from the political scene is going to be bitterly disappointed over the next few weeks and, I predict, confounded over the next few years. We have a great opportunity in this election because it is an election on Ukip's turf. It is a Brexit election."

Setting out details of what will be in Ukip's "bold and radical" manifesto, Mr Nuttall explained his party would be fighting for "the kind of Brexit people voted for", involving regaining control of borders, freedom to strike trade deals and a refusal to pay any "divorce bill" to the EU.

Ukip would promise to cut the foreign aid budget and plough the proceeds into the NHS with a commitment to keep the health service in public hands and free at the point of delivery.

The party would campaign for an English Parliament, the abolition of the House of Lords and the scrapping of the first-past-the-post voting system in favour of proportional votes for Westminster elections.

It would tackle the "repulsive" practice of female genital mutilation and insist that people must not cover their faces in public places; an effective ban on Muslim women wearing the burka.

Ukip would be the only party in the election with "a clear commitment to cutting immigration", insisted Mr Nuttall.

He said immigration rates in recent years had not only led to "a stagnation of wages, particularly in working class communities" but were also "bad for community cohesion", he said.

The Ukip leader also made clear that he would not quit if he failed to win a Commons seat.

"We are in with a chance of winning a number of seats at this election. We will be targeting sensibly in terms of financial resources and manpower.

"If we weren't successful - and by the way I don't believe for a second we will fail, we will have seats in the House of Commons after this election - would I consider my position, would I resign? No, I don't think I would because Ukip has a fantastic future ahead."

Mr Nuttall confirmed his predecessor, Nigel Farage, would play a “front-of-house role in this election”. He added: “You will be seeing him on TV, you will be hearing his dulcet tones on the radio; he will be at the forefront of the campaign."