Retailers are creaking under the sheer weight of information that the Competition Commission is calling for by next month, with some describing it as onerous and unworkable.

Twenty one of the biggest retailers and buying groups, as well as the Association of Convenience Stores (ACS), were due to have received a lengthy fact-finding questionnaire by this week, as part of the investigation into the grocery market. They have one month to complete up to 131 questions.

Tesco has already told the Commission it will not meet its deadline of 10 August and is in the process of arranging an extension. "It was not realistic," said Tesco media director Jonathan Church. "We're happy to provide the information but the volume and level it is asking for takes time to pull together."

Another major retailer said it was a "huge ask" to get the information within one month. "Even not taking into account all the questions within questions, this is a massive job."

Neil Turton, chief operating officer of Nisa-Today's, added: "The workload involved is inappropriate, unmanageable and unworkable for organisations of our type and size. If Tesco is struggling with the workload, you can imagine how onerous a task it is for us."

However, Angela Barber, trading and marketing director for Costcutter, reasoned that without the detail the review of the industry may be skewed.

"We appreciate that the Commission does need this information in order to get a view of the full trading picture. We want to do everything we can to provide the appropriate information."

She added that due to a lack of time resources, ­Costcutter staff would need to work through the questions in the evenings and at weekends.

As a result of the concerns of retailers, the Commission has now said it will consider extending deadlines.

"The main thing is that they talk to us," said a spokesman. "If there are good reasons why they need extra time to respond to some questions, or why they might not be able to respond at all, then we're prepared to be flexible."

He added: "We want to get the inquiry done well within the time allowed. And while this puts demand on the parties, it is equally the case that we're setting a demanding timetable for ourselves."

Questionnaire recipients

Tesco

Asda

Sainsbury's

Morrisons

Somerfield

Waitrose

Marks and Spencer

Iceland

Kwik Save

Aldi

Lidl

Netto

The Co-operative Group

Costcutter

Nisa-Today's

Palmer & Harvey McLane

Proudfoot Group

Spar

Booths

Musgrave Budgens Londis

Pareto Retail

ACS