Anderson was suspended from the Conservative Party over accusations of Islamophobia.
The move on Monday by Lee Anderson, a former miner who has courted controversy with his outspoken views, comes months before a national election in which Reform is expected to draw votes away from the Conservatives and in doing so threaten Sunak’s re-election bid.
Anderson’s defection to Reform, which has Brexit campaigner Nigel Farage as its honorary president and backs populist causes such as tougher immigration laws, gives the party its first member of parliament.
It also represents a blow to Sunak, given Anderson was appointed as the Conservatives’ deputy chairman last year to appeal to voters in former Labour Party-voting heartlands known as the “Red Wall”, which backed the Conservatives at the last election.
The New Conservatives, a group of MPs on the right of the Tory party who have rebelled against Sunak, said the Conservative Party was responsible for Anderson’s defection.
“We cannot pretend any longer that ‘the plan is working’. We need to change course urgently,” the group said in a statement.
“I want my country back. … We are allowing people into our country that will never integrate and adopt our British values,” Anderson said at a press conference alongside Reform leader Richard Tice, who predicted more lawmakers would join Reform.
“Reform UK has offered me the chance to speak out in parliament on behalf of millions of people up and down the country who feel that they’re not being listened to.”
Last month the Conservatives suspended Anderson after he refused to apologise for saying London’s first Muslim mayor, Sadiq Khan, was under the control of “Islamists”.
While the Labour Party of opposition leader Keir Starmer is Sunak’s biggest election rival, Reform could win over some traditionally Conservative-leaning voters.
Last month, Reform achieved its best result in one-off parliamentary contests known as by-elections, taking 13 percent of the vote in a constituency in central England, once considered a safe Conservative seat.
The party, founded in 2018 as the Brexit Party, has ruled out making any pre-election deals with the Conservatives.
The Conservatives said the party regretted Anderson’s move.
“Voting for Reform can’t deliver anything apart from a Keir Starmer-led Labour government that would take us back to square one – which means higher taxes, higher energy costs, no action on Channel crossings and uncontrolled immigration,” a Conservative spokesperson said.
Anderson quit his post as one of Conservative Party’s deputy chairmen in January to vote for amendments to toughen up immigration legislation that would revive the government’s plans to deport asylum seekers to Rwanda.
The 57-year-old supports the return of capital punishment, wants asylum seekers to be immediately returned to their countries of origin and earned the nickname “30p Lee” after claiming that decent meals could be made for 30 pence (38 US cents).
The row over Anderson’s comments about London’s mayor came at a sensitive time in British politics as Israel’s war on Gaza exacerbates tensions in Jewish and Muslim communities that have spilled into parliament.