Labour has asked the press regulator Ipso to reopen a complaint against national newspaper coverage of Jeremy Corbyn’s 2014 trip to Tunisia after the party missed a deadline to provide paperwork.

The press regulation complaint, the first on behalf of a senior serving politician, was filed in the summer. This week, however, the Guardian revealed that the Ipso investigation would not proceed for unknown reasons.

This prompted speculation that Corbyn’s team had decided against asking the regulator to rule definitively on whether the Sun, the Times, the Telegraph, the Daily Mail, the Express and Metro had misrepresented the Labour leader’s visit to the graves of Palestinian activists, and implied that he laid a wreath commemorating members of the Black September terrorist group.

It can now be revealed that the complaint was shelved after the party missed a deadline to tell Ipso that it still wanted to push ahead with the challenge to newspapers.

Labour is now asking the press regulator to make an exception to its rules and reopen the case despite missing the deadline, on the basis of the “extenuating circumstances” that officials were too busy dealing with party conference preparations and a staff member had been ill for several days.

“There is an unprecedented volume of material raised through the responses referring to multiple sources which need to be checked,” said an email by a Labour official to the regulator seen by the Guardian, explaining why it missed the deadline to say it wished to continue with its challenge.

“These all need to be sifted through and carefully considered because of the volume of the inaccurate reporting we complained against. Much of this is historical, and over the summer recess period, crucial colleagues whose input was needed were away. The deadline also fell within the party conference season.

“We do wish to progress the complaints, and would, therefore, like Ipso to investigate further.”

The party seems to have sprung into action and requested the extension on Tuesday, the day after the Guardian reported that the complaint had been shelved.

Under the Ipso process, a complaint is first put to the publication in question in the hope that it can be resolved directly without the need for further investigation. The newspapers then have a short timeframe to provide the complainant with supporting evidence for their original reporting.

If Ipso does not receive further correspondence from a complainant within 28 days, it will then write to ask whether the complainant wishes to take the case further. It appears that Labour missed the deadline to reply to this follow-up email by two days, leading Ipso to close the case.

Labour said its decision had been delayed by the fact that some of the responses from newspapers were received a month after its initial complaint: “This did not give us time to fully decide the best course of action for the complaints overall.”

A spokesperson for the Labour party said: “We stand by our complaints and are currently looking at how to progress them. But it is clearly a worrying sign of how this is being handled that confidential correspondence with Ipso has been leaked. Trust in the integrity of a press regulator and its procedures is essential if the system is to command confidence and function effectively.”

Ipso’s complaints committee will have to decide whether to allow Labour to pursue the case. The regulator declined to comment.