The rival campaign teams challenging to lead the Leave campaign in the EU referendum are to submit their formal applications to the official elections watchdog.
The Electoral Commission is charged with designating the lead campaigns for both Leave and Remain - with the chosen teams enjoying major advantages in spending, campaign broadcasts, and mail shots.
But while it is expected that only one group - Britain Stronger In Europe - will apply for the Remain designation, the Leave designation is being bitterly contested by two rival set-ups.
While both campaigns have been at pains the emphasise their cross-party support, Vote Leave is notable for the backing of Conservative heavyweights such as London mayor Boris Johnson and Cabinet ministers Michael Gove, Chris Grayling, John Whittingdale and Theresa Villiers as well as the recently resigned Iain Duncan Smith.
Against them, the rival Grassroots Out group has been stressing its popular credentials - staging a series of rallies around the country and claiming the support of 2.7 million people, including 2,000 local councillors.
It has the backing of Conservative, Labour and DUP MPs as well as Ukip leader Nigel Farage - probably the most high-profile figure on the Leave side after Mr Johnson - and the wealthy Ukip backer Arron Banks.
In deciding between them, the Electoral Commission will look at the "breadth of support" each campaign commands, how it intends to get its message across to voters, and how it plans to engage with other campaigners on the lead side.
The commission must announce its decision by April 14 at the latest.
With Britain Stronger In Europe also set to submit its official application to lead the Remain campaign, David Cameron said they had support drawn "from every corner of the UK" and from supporters of each of the main parties and those who support none.
"By contrast, those campaigning to leave the EU are divided into at least two camps, who can't agree on very much," he said.
"In fact, the only common ground is their inability to tell the British people what would happen if we left the EU. They have failed to answer reasonable questions about what would happen to jobs, prices or our country's security if Britain left the EU."
Britain Stronger In Europe also released a study from the London School of Economics which, it said, calculated that prices in the UK were £200 lower per household per year due to EU free trade agreements, bringing savings of £5.3 billion a year for the country.
The report also estimated that future EU trade agreements with the US and Japan, would be worth an additional £238-a-year to households in the form of cheaper prices.
The claims were challenged by Vote Leave spokesman Robert Oxley who said independent research had shown that the EU had actually increased costs for consumers.
"That's on top of the £350 million we all have to fork out for Brussels each week," he said.
"The EU has been very bad at negotiating free trade deals hurting jobs and the economy. That's why it's safer for Britain to take back control of negotiating its trade deals so it can prosper outside of the EU."