Another major tactical voting site is launching designed to deliver an anti-Brexit parliament, with three separate campaigns now advising voters on how to help stop a Conservative majority.

The Remain United site, led by businesswoman Gina Miller, who led the case against the government’s suspension of parliament, will make recommendations in constituencies where it believes tactical voting will make a difference.

However, while tactical voting could play a key role in shaping the result of a volatile election, there are already concerns that the three sites disagree on which party voters should back in dozens of seats around the country.

Under current plans, Miller’s site will back about 50 Liberal Democrat candidates. Another major site already up and running, run by the Best for Britain campaign, recommends about 180 Lib Dems. Remain United’s model suggests that the Lib Dems are likely to win only 33 seats if there is a significant tactical voting drive. A third site, run by the People’s Vote campaign, also launched this weekend with its own set of recommendations.

Miller told the Observer that she had tried to see if pro-Remain campaigns could recommend one tactical voting site, but it had not proved possible. She said that her recommendations were based “purely on the data” and that the recommendations would be updated as the campaign progressed.

“If we don’t tactical vote, what we will end up with is a Tory majority that is going to pursue a Brexit plan that is no longer theoretical,” Miller said. “It is not as though we don’t know what is going to happen. We know the direction of travel. If you look through the withdrawal agreement, it significantly shifts accountability. Executive power increases, while parliamentary scrutiny decreases.

High-quality data is expensive and understanding it is time-consuming … inevitably, it upsets people in different parties Naomi Smith, CEO, Best for Britain

“What is going to upset a lot of parties is that we have no emotional attachment. This is going to be driven by the data and the results are quite different to the results out there already. I can foresee the Lib Dem seats going up [by] a handful, but it’s not going to be anything particularly dramatic.

“Our research is showing that between 45% and 55% are going to vote tactically. We’re starting from a different place to the 2017 election. But you can’t just go on past polling data or down party lines. We’ve used as many layers as possible.”

Naomi Smith, chief executive of Best for Britain, said electoral law made cooperation hard. “There are very strict rules under electoral law, which restrict campaigns from working jointly. But there’s also a question of data. We are a data-driven campaign, and we’ve used the latest and best seat-level data to make recommendations. High-quality data is expensive and understanding it is extremely time-consuming, requiring hard work from a committed team. Inevitably, it upsets people in the different parties, including some of our friends. But we know this is our best shot at stopping Boris Johnson and the damaging Brexit he wants for our country.”

Miller is going to publish both her site’s methodology and data, to ensure as much transparency as possible. Her site’s research suggests that the Tories are currently on course to win a majority, with 347 seats. However, it says that, with a tactical voting drive, a narrow majority for a coalition of Labour, Lib Dems and the SNP could deliver a second referendum. Her model – which uses polling that is then combined with demographic data and past voting to come up with recommendations – suggests tactical voting could reduce the Tories to 309 seats, with Labour on 233 and the Lib Dems on 33 seats.

The People’s Vote campaign has been plagued by internal splits and a boardroom battle for control in recent weeks. Its site’s recommendations are based on analysis of private and public polling data, along with information from election results at all levels going back several years.