Labour has become an institutionally racist organisation through its failure to deal with antisemitism, one of its MPs has argued. Chuka Umunna said the party needed to make far greater efforts to tackle the problem.

Asked on Sky News’s Sophy Ridge on Sunday programme whether he believed Labour was now institutionally racist, a term used in the Macpherson report on the Metropolitan police after the murder of Stephen Lawrence, Umunna said he did.

He said Labour now clearly fitted “the definition of institutional racism as outlined by Sir William Macpherson in the Macpherson report”.

Umunna said: “It’s very painful for me to say that. Part of the reason that I joined the Labour party, that my family supported the party, was because it was an anti-racist party. I think the failure to deal with the racism that is antisemitism is particular, and clearly is a problem.”

The Streatham MP, one of the leading backbenchers from the party’s centre-left, previously said people with his views were being forced out of Labour under Jeremy Corbyn, and he urged the leader to “call off the dogs”.

Those words prompted an angry response from the shadow chancellor, John McDonnell, who branded his choice of words “grotesquely offensive” to party members. “Our party members are not dogs,” he said.

Umunna said calls for mandatory re-selection processes for Labour MPs and no-confidence votes by local parties against members such as Frank Field and Joan Ryan were an attempt to “hound them from office”.

Ryan, who chairs the Labour Friends of Israel group, lost a confidence vote in her Enfield North constituency on Thursday, as did Gavin Shuker in Luton South.

Umunna said: “The real issue here is that the extraordinary has become the new normal in the Labour party. It is not normal what we are seeing, where you have a Labour MP who is well respected on both sides of the house like Frank Field resigning the whip because of the culture in the party and because of antisemitism.

“It’s not normal to have two Jewish Labour MPs give impassioned, moving speeches in the House of Commons outlining in detail the racism to which they have been subject, including by supporters and members of the Labour party.”

Jennie Formby, Labour’s general secretary, rejected Umunna’s view. “The Labour party, which I’ve been a proud member of for 40 years, is not institutionally racist,” she wrote on Facebook. “To suggest it is shows no understanding of either the work or the history of our party.”

Dawn Butler, the shadow equalities minister, said Umunna’s comments were “disappointing”. She tweeted: “I have literally spent all of life fighting racism. And today is a sad day. UK Labour is NOT institutionally racist.”

The Labour chairman, Ian Lavery, told the Ridge programme that while MPs should be “accountable”, there was no concerted effort under way to purge the party of centrists.

Lavery, the MP for Wansbeck, said: “These votes of no confidence hold no water, they are basically a statement from the constituencies. These aren’t individuals being targeted, they are being challenged.”

He added: “When people think that they are being unfairly challenged they need to be accountable to the people that they represent.”

Lavery conceded that tackling antisemitism in Labour had been “a massive challenge”, but he said progress was being made on investigating complaints. However, he said he had no detailed information on this, for example on the number of members expelled.

“But we’ve got a huge challenge ahead with the Jewish communities,” he said. “We understand fully the hurt which has been caused within the Jewish communities. We want to make amends.”

Lavery joined McDonnell in condemning Umunna’s use of the phrase “call off the dogs”, saying: “Calling anybody a dog is absolutely outrageous in the extreme.”

Umunna said he was baffled by the criticism: “The phrase that I used is a metaphor, it’s a figure of speech.”

Asked why he did not simply leave the party, Umunna said: “Because I want it to change. There’s institutional racism in many institutions in our county, and if you simply leave the field, as it were, instead of try and argue and see change through in an organisation, then I’m not sure you always make progress. That’s the reason why.”

He played down the idea of a new centrist party: “I’ve been a member of the Labour party for more than 20 years and I joined with the intention of never leaving it. I want to be a member of the Labour party on my deathbed, and I hope that is the case.”