Concerns are mounting over whether Labour conference – due to open in less than a month – will go ahead after a major security company rejected a late offer to cover the event.

Labour approached G4S, which has provided security numerous times previously, earlier this week in an effort to solve the crisis that has thrown the annual conference into doubt. However, the company says it does not have sufficient time to make arrangements ahead of the September 24 start date. Working with G4S would have meant reversing a decision by the NEC earlier this year to boycott the firm.

A spokesperson for G4S yesterday said: “Safety for delegates and our staff is our priority and at this late stage and with our teams committed elsewhere, we are not in a position to step in and provide security for the conference.”

The only company to put in a bid for a security contract with Labour is Showsec – although an ongoing trade union dispute has led GMB to threaten to picket the party conference if such a deal goes ahead. Many party members and trade unionists would refuse to cross a picket line, throwing the conference into further chaos.

Another option, bringing in local police to provide security, also appears doubtful. As well as being incredibly costly, Merseyside Police are likely to be unsure about taking on the task at such short notice – a Liverpool FC home match on the day of the leadership contest announcement could also put pressure on the local police service. A spokesperson for Merseyside Police said that them stepping in is “not an option at the moment”, as they have not been approached by Labour.

The problem has heightened tensions across the labour movement, with leaked letters between GMB and Unite chiefs revealing growing unhappiness.

GMB general secretary Tim Roache initially wrote to Unite’s Len McCluskey, warning him not to “support a Labour party conference staffed by a union-busting firm”.

“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,” he wrote. “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.”

In his reply, which also came to light in the press, McCluskey said that any problems around conference arrangements were the fault of Labour’s General Secretary, Iain McNicol. “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,” McCluskey wrote.

The Unite boss also reacted sternly to the implication that his union was seeking to undermine the GMB.

“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.”

McNicol has also found himself at the centre of a row with the Jeremy Corbyn leadership campaign, which yesterday claimed that “party officials” were carrying out a “purge” of Corbyn supporters after another trade union general secretary, BWAFU’s Ronnie Draper, had his Labour membership suspended.