The leader of the Unite union, Len McCluskey, has said Labour’s conference may not go ahead if the row over its security is not resolved, and said the blame would be shouldered by the party’s general secretary, Iain McNicol.

Labour is still no closer to resolving the security issue for its annual conference in Liverpool next month after the national executive committee voted to boycott longstanding provider G4S.



Though other security firms were approached, Showsec was the only company to bid for the contract. Showsec is in dispute with the GMB over its refusal to sign a union recognition agreement, and talks between the union and the company have broken down.



Last week, GMB’s general secretary, Tim Roache, implied in a leaked letter that McCluskey was interfering in the dispute, and said his union would picket the conference were Showsec to win the contract, forcing most Labour MPs and members either to cross a picket line or not attend



Now McCluskey, in a letter to Roache seen by the Guardian, said it was clear the conference would not go ahead following a GMB boycott.

“It is the responsibility of the general secretary of the Labour party, Iain McNicol, to implement decisions or to deal with any problems that may arise,” McCluskey wrote. “I am astonished that we are only four to five weeks to conference and that he has not done so.

“It is quite evident that in the event of a GMB boycott of conference, it simply won’t proceed and the blame would lay squarely at the feet of Iain McNicol.”

In a further sign of the deteriorating relationship between the two unions, McCluskey denied Unite had attempted to interfere in the dispute. Unite has backed the Labour leader, Jeremy Corbyn, in the leadership election while the GMB supports his rival, Owen Smith.

In the letter from Roache to McCluskey, leaked to PoliticsHome last week, the GMB boss said: “As you know, we are the dominant union in the security sector, and we will not tolerate any attempt by other unions to find a solution by dreaming up an inferior recognition agreement.

“If Showsec do not sign our standard agreement, GMB will be forced to boycott and picket conference and many friends and colleagues will find themselves unable to cross our picket line.”

Roache said he hoped McCluskey would either allow urgent discussions with G4S or make it clear he would “support a Labour party conference staffed by a union-busting firm”.



In his reply, McCluskey said his union also had an interest in the security sector but denied he had had any contact from the party over the matter.



“I am somewhat perplexed by the nature and content of your letter as it appears to imply that Unite is somehow responsible for this current shambles and the tone set by your expression ‘will not tolerate’ other unions setting inferior standards, infers that there is a dispute between Unite and the GMB,” McCluskey wrote.



“Who are the ‘other unions’ you are talking about? Indeed, your letter, which was leaked to the media, has been seen by some journalists in this light and stories have emanated from it.”

Labour’s NEC voted to boycott G4S, which has a union recognition agreement, amid concerns over controversies at various detention facilities run by the company.



Alice Perry, a member of the NEC, said it was crucial Labour members were not asked to attend a conference where security was provided by a company that did not accept trade union recognition.



“I hope that everything can be resolved swiftly,” she said. “It is important that our conference security provider recognises trade unions and meets the ethical standards the Labour NEC expects from all our contractors.”



McNicol has been under increasing pressure following the party’s decision to fight a legal challenge by five new Labour members over an NEC ruling that blocked more than 100,000 new members who joined in the past six months from voting in the leadership ballot.

The party successfully took the case to the court of appeal, a judicial ruling that Corbyn said was “the wrong decision – both legally and democratically”.

Allies of Corbyn, who see McNicol as working closely with the deputy leader, Tom Watson, have suggested the general secretary’s position could be under threat if Corbyn is re-elected. More than 100 former and retired members of Labour party staff wrote to the Guardian last week calling for attacks on party staff to cease.



The letter says despite the long hours, stressful working environment and little job security, party staff “remain completely loyal to the party and to their employers, and the least they are entitled to expect is some loyalty and respect in return … To hear members of the Labour party attack their own employees is depressing; to hear talk about ‘clearing them out’ is unacceptable; to hear such statements from the most senior level is intolerable”.



Corbyn has since written to staff to address their concerns, saying they “must not be used as a political football by anybody within the party … I hope you all feel that if you are put in difficult or unacceptable circumstances, you can raise the issue with your line manager, other senior member of staff, or your trade union representatives.”



Labour has insisted the conference security issue will be resolved, but said it would not comment on commercial matters.