Only a few dozen people are expected to claim benefits using the government's flagship new universal credit scheme when it is launched on Monday and the total number who will qualify will be limited to 300 a month, in a radical scaling back of the pilot.

The tiny scale and extremely soft nature of the launch has prompted concern from charities and opposition politicians over how useful the trial will be in exposing weaknesses in the complex new welfare scheme, ahead of its nationwide rollout, which is due to begin in October.

Only the most straightforward benefit claims will be included in the trial, with any cases with a hint of complexity excluded. Tameside council, (which is the only place piloting the benefit after pilots in three other areas were postponed last month), expects just 300 people to start claiming the benefit in the first month.

Tameside told the Guardian that only single people with no children, newly claiming a benefit, will be told to claim universal credit, stipulating that they must also be fit for work, not be claiming disability benefits, not have any caring responsibilities, must not be homeless, living in temporary accommodation, and must have a valid bank account and national insurance number.

The government promises universal credit will simplify the benefits system by bringing all the strands of support – from housing benefit to tax credits – into one payment, but there has been consistent concern from Labour that the development of the computer system it relies on has not gone smoothly and that the targets for implementation have been discreetly pushed back. In the space of six months, there have been four different directors overseeing the development of the IT systems.

The critical test of whether the newly-designed computer programme can cope with the pressure will remain unanswered at this stage because of the small quantity of individuals likely to be logging on to the system.

Nigel Keohane, deputy director of the Social Market Foundation, a cross-party thinktank, said the department appeared to have removed any difficult scenarios from the initial pilot. "In essence, they are testing the simplest group and only a very small number of them."

The shadow work and pensions secretary, Liam Byrne, said: "Universal credit is a good idea that builds on Labour's tax credits revolution that made sure work paid more than benefits for millions of people. But in three years it's gone from a flagship programme to a car crash. This pathfinder is a pathetic joke. This project is now late, over budget, the IT system is a mess and half the cabinet will tell anyone who will listen that the scheme is doomed. We must have a rescue plan – and fast."

There was particular concern that no benefits related to children would be involved in the pilots. Matthew Reed, chief executive of The Children's Society said: "It is very worrying that families with children are not being targeted in its testing phase. This means any potential problems they may face will not be picked up in time for its rollout across the country from October.

"Welfare reforms on this scale will have a significant effect on families with children. If universal credit is to be fair and effective it must be fully tested and any problems resolved ahead of its implementation."

The national rollout of the universal credit begins in October and the plan is for it to be fully operational by 2017.

A spokesperson for the Department for Work and Pensions said the whole point of the pilot was to start small. "This is the very first step. We are testing how our systems work in an online environment and how claimants in a live environment react to it. We are deliberately not doing a big bang – that is not how you do a big project sensibly," he said.

The reason no claimants with children were included at this stage was to ensure that the first step was simple. Children and families would also be excluded from the October national launch, for the same reason, he said, and there was no fixed date for when they might begin to be incorporated into the system.

While there was agreement that it made sense to start with a small sample for preliminary tests of the system, the extremely small size of the cohort being tested (estimated at around a fifth of those due to be in the pilots, before three were postponed) has prompted concern about whether it will accurately flag up problems before the national rollout in October. Jobcentres in Wigan, Warrington, and Oldham were also due to launch pilots at the end of April, but are now not due to start taking limited numbers of claims until July.

Alban Hawksworth, benefits specialist at the welfare advice charity Turn2us, said the pilot was not a rigorous test of the system.

"The point of a successful pilot is an opportunity to learn," he said. The scale of the first step was "calling into question the validity of the pilot as something to learn from." His analysis of benefits statistics suggested that the pilot would be dealing with just 0.5% of all potential benefits claimants in the area.

Tameside council has installed a Citizens Advice hub in its offices to help new claimants with the process of claiming the benefit online, but staff are not expecting to help large numbers initially.

Nigel Morgan, district manager of Tameside CAB, said he had been trained on the new system over the past fortnight. He described it as user-friendly, although he said that it took about 45 minutes to fill in the forms, and noted that there was no save function, which meant that if claimants needed to go home and get more information, they had to begin all over again.

He said the first batch of potential claimants would be people recently made unemployed, who were likely to be computer literate, which would also make things easier for staff. Advisers were on standby to help people budget for new monthly payments.

David Heyes, Labour MP for Ashton-under-Lyne, said he would be working closely with the council's welfare offices and with Citizens Advice, and would be trying to monitor the progress of the pilot. "It is just too early to say whether this is going in the right direction. I want to be sure that the evidence from the pilot is going to be publicly available and treated objectively. There is a suspicion that they will hold the information close to their chests," he said.