POLICE are investigating senior figures in the Vote Leave campaign after the group was fined for breaking strict electoral rules – sparking fresh calls to re-run the EU referendum.

Winston Churchill’s grandson, Sir Nicholas Soames, declared the British electoral system should be “blown up and started all over again” after it emerged the official campaign for leaving the EU exceeded spending limits by almost £500,000, flouting electoral law.

Investigators found "significant evidence" of joint working between Vote Leave and another campaign group, BeLeave, which was founded by Darren Grimes, then a 23-year-old fashion student.

The former was fined £61,000.

Sir Nicholas told MPs “one of the great glories of this sadly now diminished country was our electoral and democratic system, and this example today is gross”.

He added: "If we are to retain the integrity and the trust of the voting public, the whole damn thing needs to be blown up and started all over again."

His drastic call came as campaigners demanded action to “rein in the wild west of election campaigning”, urging full transparency and unlimited fines for those who break the rules.

Kyle Taylor, director of Fair Vote UK, said: “This is much bigger than Brexit: breaking election law is undermining the very fabric of how we believe our society should make rules. We are sleepwalking into a future I don’t want to be a part of.”

Sir Nicholas spoke out following an urgent question at Westminster from Labour's Chuka Umunna, who condemned the revelations as an “affront to our democracy”.

Mr Umunna raised concerns “foul play” had affected the result, which saw a 4 per cent gap between Leave and Remain. Vote Leave overspent by just under 8%.

SNP Brexit Secretary Mike Russell had earlier insisted the vote to leave the EU was “obtained, at least in part, by breaching electoral law”.

He argued another vote was now “essential”, providing “there are safeguards for Scotland in terms of possible outcomes” – suggesting another independence referendum if Scotland voted to stay in the EU.

Labour has called for ministers and formers ministers who were key figures in Vote Leave – such as Michael Gove and Boris Johnson – to be probed for potential misconduct.

Former minister David Lammy said Vote Leave had cheated on spending rules and urged a new vote on whether the referendum result was now void.

Criticism also flooded in from the Conservative benches, with senior MP Sarah Wollaston insisting “consequences must follow” from the findings.

She added: “We cannot have confidence that this referendum was secure and it should be re-run."

The Electoral Commission found Vote Leave failed to declare money it spent with controversial data firm Aggregate IQ.

Its probe centred on a donation of almost £680,000 made by Vote Leave to BeLeave, a youth Brexit group founded by Mr Grimes.

BeLeave "spent more than £675,000 with Aggregate IQ under a common plan with Vote Leave", inspectors found. This was not declared and took the latter over the legal spending limit.

According to whistleblowers, the cash was used to pay for targeted messaging services on Facebook and other social media.

Bob Posner, the Electoral Commission’s director of political finance, said the findings amounted to “serious breaches of the laws” which ensure “fairness and transparency at elections and referendums”.

He said Vote Leave had “refused to co-operate, refused our requests to put forward a representative for interview, and forced us to use our legal powers to compel it to provide evidence”.

Mr Grimes was fined £20,000 and referred to the Metropolitan Police along with David Halsall, the responsible person for Vote Leave, "in relation to false declarations of campaign spending".

In a statement on Twitter, Mr Grimes said the fine was "entirely disproportionate and unjustified".

He added: "Politicians say they want young people to engage with politics. I was 22 when I got involved in a referendum I felt passionately about.

"I did nothing wrong. I have been persecuted for over two years by powerful people for nothing more than engaging in the democratic process and having the temerity to be on the winning side. It has been appalling for my family."

It comes just months after Brexit campaign group Leave.EU was fined a record-equalling £70,000 and its chief executive Liz Bilney referred to police over its spending during the referendum campaign.

A Vote Leave spokesman said the Electoral Commission's report contained "a number of false accusations and incorrect assertions that are wholly inaccurate and do not stand up to scrutiny".

Cabinet Office minister Chloe Smith said she could not comment on an ongoing police investigation. But she insisted the Government was “getting on with delivering the result of the referendum”.

A spokeswoman for the Met Police said the findings would be assessed by officers from its Special Enquiry Team.

Meanwhile, the SNP said the debacle should renew focus on a highly controversial £435,000 donation made by former Scottish Tory vice chair Richard Cook during the EU referendum.

The money was given by the mysterious Constitutional Research Council, which Mr Cook chaired, to the DUP in Northern Ireland, where donations are traditionally secret.

It was then used by the DUP to buy pro-Brexit newspaper advertising outside Northern Ireland, principally in London – sparking concerns it had exploited a loophole in electoral law.