Labour has apologised to the voters of Rochdale for not fielding a candidate after the by-election triumph of George Galloway, following what Rishi Sunak described as “one of the most divisive campaigns” in recent times.

Sir Keir Starmer’s party warned Mr Galloway will stoke “fear and division” and said he “only won because Labour did not stand” after it dropped its candidate Azhar Ali for suggesting Israel was complicit in Hamas’ October 7 attack.

Mr Galloway, one of Britain’s most divisive politicians, swept to victory in the greater Manchester seat, gaining almost 40% of the vote in a contest mired in chaos and controversy and dominated by the Gaza conflict.

The former Labour and Respect MP, who now leads the Workers Party of Britain, took aim at the Labour leader in his victory speech focused on Palestine.

Mr Galloway said: “Keir Starmer, this is for Gaza. You have paid, and you will pay, a high price for the role that you have played in enabling, encouraging and covering for the catastrophe presently going on in occupied Palestine in the Gaza Strip.”

He also declared “Labour is on notice” and a “shifting of the tectonic plates” away from Sir Keir’s party.

Labour had been expected to win the by-election triggered by the death of Labour MP Tony Lloyd, but its campaign was thrown into disarray by a leaked recording of Mr Ali’s inflammatory remarks about Israel, seeing Mr Galloway become the firm favourite for the seat.

Mr Ali remained listed as the Labour candidate as the party’s decision came too late for ballot papers to be changed.

Sir Keir restated Labour’s apology on Friday, telling broadcasters: “Galloway only won because Labour didn’t stand a candidate.

“I regret that we had to withdraw candidate and apologise to voters in Rochdale.

“But I took that decision. It was the right decision. And when I say I changed the Labour Party, I mean it.

“Obviously we will put a first-class candidate, a unifier, before the voters in Rochdale at the general election.”

Ellie Reeves, Labour’s deputy national campaign co-ordinator, told Sky News: “George Galloway is someone who stokes up division and fear. This isn’t how we would have wanted this by-election to play out.”

She also suggested Labour would not change its position on the Middle East war, calling for an “immediate humanitarian ceasefire” with some caveats.

Left-wing pressure group Momentum said Sir Keir’s “failure to stand with Gaza in its hour of need left the door open for George Galloway”.

“To avoid any more damaging repeats, Starmer should end the factional abuse of Labour’s selection processes and stand up for an immediate, permanent ceasefire in Gaza.”

Mayor of Greater Manchester Andy Burnham said: “Labour has let the people of Rochdale down because it didn’t give them a good enough choice at the election held yesterday.

“It’s right that the leader of the party has apologised to the people of Rochdale but we will now work with him and his team to make sure there is some reflection on what has happened, some honesty, so that we can all come together and work together to regain the trust of the people of Rochdale.”

The Rochdale campaign was beset by controversy and claims of intimidation and divisive tactics.

Speaking during a visit to Scotland on Friday, Prime Minister Rishi Sunak told reporters: “It was very concerning to see the reports of intimidation through the by-election, and by all accounts one of the most divisive campaigns that we’ve seen in recent times.

“I’m pleased the Conservative Party was the only party to run a really positive campaign focused on local issues with a great local candidate, Paul Ellison.”

But Mr Sunak’s Tories performed poorly, with polling expert Sir John Curtice telling BBC Breakfast the Conservative vote was “down by 19 points – it’s the biggest drop in the Conservative vote in a Labour-held seat in a by-election in this parliament”.

Conservative former minister Sir Sajid Javid tweeted: “An awful by-election outcome for my home town of Rochdale and the country.

“Exceptional circumstances, and a very low turnout. But still a vote for hateful, toxic and divisive politics. Bigger lessons must be learned.”

Mr Galloway’s majority of 5,697 votes amounted to 18.3% of the total, on a turnout of 39.7%, a little higher than the two recent by-elections in Wellingborough and Kingswood.

The surprise runner-up was independent candidate David Tully, a political newcomer, who secured more than 6,600 votes.

Reform UK candidate, former Rochdale MP Simon Danczuk, said there were “lots of examples of intimidation” during the campaign.

He told Times Radio: “We spent a lot of time dealing with intimidation and significant problems that clearly came from Galloway’s supporters.”

Mr Danczuk, who once represented the seat for Labour but was barred from standing for the party in 2017 after he admitted sending “inappropriate” messages to a 17-year-old girl, secured a little more than 6% of the vote, coming sixth behind Mr Ali, the former Labour candidate.

Independent candidate William Howarth agreed that there had been an “element of intimidation” during the campaign.

But Mr Galloway denied his supporters had engaged in any intimidation, and claimed on Sky News that Mr Tice had invited him to be a Reform UK candidate in a recent by-election.

A spokesperson for the charity Campaign Against Antisemitism said: “George Galloway has an atrocious record of baiting the Jewish community.”

They added: “Given his historic inflammatory rhetoric and the current situation faced by the Jewish community in this country, we are extremely concerned by how he may use the platform of the House of Commons in the remaining months of this Parliament.”

The Board of Deputies of British Jews said the victory of George Galloway, a “demagogue and conspiracy theorist”, marked “a dark day” for the UK’s Jewish community.