Matthew Elliott, the former chief executive of Vote Leave and founder of the rightwing pressure group the TaxPayers’ Alliance, could be named as an adviser to Sajid Javid at the Treasury, according to reports.

After managing the new chancellor’s failed campaign for prime minister, Elliott joins Dominic Cummings, the former head of the Vote Leave campaign who is now one of two senior advisers to Boris Johnson, at the heart of policymaking.

Elliott, who is also editor-at-large of the pro-leave news site Brexit Central and political adviser to the investment bank Shore Capital, is likely to come under pressure to step down from his other roles owing to potential conflicts of interest.

A Treasury spokesman said: “We do not comment on speculation.”

The legal head of the pro-Brexit campaign, which broke electoral law by overspending, Elliott registered the company Vote Leave Ltd and filed its legal documents with the Electoral commission. The group was responsible for the infamous campaign message: “Give our NHS the £350m the EU takes every week.”

An opponent of a soft Brexit, Elliott told the New Statesman in a rare interview last year: “We won a crucial battle but we could lose the overall war ... I’m disappointed that the leave side left the battlefield after the referendum.”

Prior to his campaigning for the UK’s exit from the EU, in 2004 Elliott launched the TaxPayers’ Alliance, an organisation campaigning for minimal government and low taxes. It claims to be independent and grassroots but has received hundreds of thousands of pounds in foreign donations, often from undeclared backers. He served as its chief executive until 2014.

After the Brexit vote in 2016, Elliott was appointed senior political adviser to Shore Capital, which specialises in “alternative asset management”. He told the Times following the announcement: “Business, the economy and investment are all now so interlinked with both domestic politics and international affairs that I am delighted to be joining Shore Capital to help their clients understand the changing political situation both in the UK and overseas.”

He is married to Sarah Elliott, the chair of Republicans Overseas UK who has been effusive in her praise for Donald Trump.

Elliott’s role as a member of Conservative Friends of Russia, which has been accused of uncritically championing Vladimir Putin, has also come under scrutiny. The Observer reported that Elliott was targeted by a man believed by the Home Office to be a Russian spy in 2012, or possibly prior to that date.

At the Treasury, it is unclear whether the free-marketer Javid, a former vice-president at Chase Manhattan bank, will oppose Johnson’s spending plans if his pledges are delivered upon.

On Wednesday evening, Elliott congratulated Javid on his appointment. He tweeted: “He will be a great chancellor and ensure that the country is well positioned to reap the economic rewards of Brexit.”