Labour has become institutionally anti-Semitic, and any split in the party will be the responsibility of Jeremy Corbyn and his supporters, Chuka Umunna has told party members in his Streatham constituency. The former shadow cabinet member refused to rule out leaving the party in his remarks, which were made in a letter to party branches.

Umunna’s message came in response to a motion asking him to clarify whether he had been involved in any discussions around a new political party and if he planned to join one.

In the letter, Umunna described “Brexit and anti-Semitism” as the two biggest divides in Labour politics, and dismissed claims that the anti-Brexit campaign, of which he is a lead member, is a forerunner to a party split as “patently absurd” as the campaign’s breadth of support demonstrates that it would be unable to develop into a new party. He accuses the line of being peddled by “Brexiteers” keen to spread suspicion about cross-party alliances.

But the strongest section of Umunna’s letter is reserved for the matter of anti-Semitism within the Labour Party. After listing the Macpherson Report’s criteria for institutional racism, Umunna says “it is beyond doubt that Labour, as an institution, meets these criteria insofar as the Jewish community is concerned”.

The full letter is below.

I have been contacted asking about Westminster speculation and gossip around a split in the Labour Party or the launch of a new party. I cannot get to every branch meeting this evening so I have written to you all to make my position clear and to set the record straight as soon as possible.

It seems there are two issues which are fuelling this speculation of a split in the Labour Party – Brexit and anti-Semitism in the Labour Party.

First on Brexit, there is the false claim that the People’s Vote campaign – of which I am a leading voice – is the forerunner to the creation of a new “centrist” party. This is ludicrous nonsense. The People’s Vote campaign involves activists and politicians from the three main parties, the Green Party, the Scottish National Party, Plaid Cymru, and those of no particular party affiliation. The idea that Caroline Lucas’s Green Party, politicians from the nationalist parties in Scotland and Wales and figures such as myself from the main parties in England are about to come together and create a new, big-tent party is patently absurd.

This is also an idea being promoted by Brexiteers who dislike the effective cross-party working there has been in and outside of parliament to thwart efforts to pursue the most extreme of Brexits.

A few local members have written to me saying that they are unhappy with my comments about anti-Semitism in the party, and accused me of using this issue to attack Jeremy. The speeches of my colleagues Luciana Berger and Ruth Smeeth in April set out in horrifying detail the anti-Semitic abuse they have received – including from supporters of the leader. But the issue of racism, something which has affected me and my family, in our party is bigger than one individual.

Jewish community leaders have complained that allegations of anti-Semitism have not been properly dealt with, and the row that took place throughout the summer over the International Holocaust Remembrance Alliance definition of anti-Semitism shows that we have more work to do as a party to rebuild trust with the Jewish community. Local Jewish members have also raised their concerns about this with me.

In short, the Jewish community has been subject to different treatment by our party these past few years – that is discriminatory. The Macpherson report, published after the murder of Stephen Lawrence, defined institutional racism as “the collective failure fo an organisation to provide an appropriate and professional service to people because of their colour, culture or ethnic origin”. It said this “can be seen or detected in processes, attitudes and behaviour which amount to discrimination through unwitting prejudice, ignorance, thoughtlessness and racist stereotyping which disadvantage minority ethnic people”. Based on its actions (or failure to act) it is beyond doubt that Labour, as an institution, meets these criteria insofar as the Jewish community is concerned – something which should shame every member of our party.

Part of the reason I joined Labour was because it was believed to be anti-racist, which is why recent developments sicken me and I have not hesitated publicly to say so. This not something I am proud to say or want to admit. But if we are to be truly anti-racist party, then we must confront all forms of racism, including anti-Semitism, wherever we find it – even in our own backyard. We must do that, no matter how hard that may be and no matter who the leader of our party is.

I have not made plans to launch a new political party. I am a Labour member and a Labour member of parliament. I do not want this change, and ultimately I hope the people that will decide if I remain a member of parliament are the constituents who elect me.

The Labour Party has always been a broad church of views, opinions and traditions. For the Labour Party to continue to be a radical and transformational movement, I firmly believe that must remain the case. The Labour Party is a democratic socialist party – it says that in Clause Four of our party constitution – and so the centre-left tradition that is part of our party must be respected, rather than regularly attacked and our loyalty constantly questioned, as is so often the case.

We can disagree on issues, policies and ideas while remaining comradely and respectful of each other. Having been a member for over 20 years, I have been sad to see the change in tone and an increase in hostility in our local party meetings in the last few years. As a party we should be able to discuss issues and ideas without it being a test of loyalty to the party leadership or a particular faction. Disagreements over the party’s Brexit policy, for example, do not constitute a challenge to Jeremy’s leadership or an attempted coup. They are a principled disagreement on policy. Such differences have always been historically respected, as was the case when Jeremy broke the Labour whip on 428 occasions during Labour’s time in power and under several Labour leaders.

I don’t think it can be denied our party faces significant challenges, but they are not insurmountable. The Labour Party leadership has a responsibility to ensure not only that the party remains a broad church, but one of tolerance, respect and comradely conduct. If they do not, and the stories we hear of bullying, abuse and intimidation online, in branches and in CLP meetings continue, then people will be forced out of the party and the movement not through their own choice, but because they no longer feel welcome. The issue of whether there is a split in the Labour Party or whether members leave, is therefore ultimately in the hands of Jeremy Corbyn and the Labour Party leadership.