Home Office officials are being rewarded with shopping vouchers for helping to ensure failed asylum seekers lose their attempt to stay in the country, new documents reveal.

Official guidance obtained by the Guardian shows that immigration staff have been set a target of winning 70% of tribunal cases in which asylum seekers are appealing against government decisions that they should leave the UK.

These officers are also incentivised by Home Office reward schemes involving gift vouchers, cash bonuses and extra holidays, according to information received under freedom of information laws.

Asked what rewards were given to presenting officers and case owners in the fields of asylum and immigration, the department confirmed high-street vouchers for £25 or £50 were handed out to "recognise positive performance over a short period of time", including when officers "exceed their casework targets for a month".

Critics said it was a new low for officers to be rewarded for outcomes that meant asylum seekers being asked to leave the UK for countries where they claim to be facing persecution or war. The incentives undermine confidence in the fairness of the system, they say.

But the Home Office said the "success of officers in upholding asylum decisions" was only one of a range of criteria used to monitor staff performance. It is understood officers are also judged on the effectiveness of their presentation, the strength of their cases and compliance with official guidelines before getting rewards.

A Home Office spokesman declined to say how many vouchers in total had been given out to asylum and immigration staff. However, a parliamentary answer reveals 11 high street shopping vouchers for £25 have been given out solely to presenting officers in asylum cases since July 2012 as a "one-off recognition of individual performance at court". Mark Harper, a Home Office minister, said no vouchers had been issued purely for winning cases at the immigration tribunals as several factors are taken into consideration. "Presenting officers' performance is assessed equally by reference to other relevant factors that include the quality of preparation and advocacy," he said.

"Presenting officers' performance is managed in accordance with the same performance management policy that applies to all Home Office staff. Where an officer's overall performance is judged to be unsatisfactory, the Home Office's poor performance procedure may be instigated."

Sarah Teather MP, a prominent Liberal Democrat and former minister, said schemes such as this one "completely undermine any sense that the system will give a fair hearing to those who come here seeking sanctuary from war and persecution".

"If the Home Office are really giving out shopping vouchers for officers who help ensure asylum seekers lose their appeals that is a new low," she said.

"We have also seen before that setting success targets incentivises Home Office lawyers to delay those cases they are likely to lose at the last minute. Not only does this cost the taxpayer money through court costs and ever-increasing backlogs, but it leaves asylum seekers in a devastating state of limbo.

"If the Home Office wants to reduce the number of appeals they lose, they should improve the quality of the decisions they make in the first place."

In response, a firm of immigration lawyers said it was considering a legal challenge because voucher rewards for casework targets were a "clear incentive to bad practice".

James Packer, of Duncan Lewis solicitors, said: "The Home Office expects its officials to win a specific number of appeals and can reward them if they exceed their 'target'. This is a clear incentive to bad practices. I am especially dismayed to discover that a mandatory success rate of 70% applies in asylum appeals where people's lives are at stake. We believe these measures are unlawful as well as immoral and have written to the Home Office making it clear that we will bring a legal challenge unless these incentives are withdrawn."

Human rights organisations said they had evidence that some asylum seekers whose cases were rejected were tortured or otherwise persecuted on return to the destination country.

Emma Mlotshwa, coordinator of the charity Medical Justice, which works to protect the health of immigration detainees, said: "This is deeply disturbing. We have cases of people who have survived torture, claimed asylum, had their cases rejected and have then been forcibly removed from the UK.

"On arrival in their home country they have been tortured again. The fact that some Home Office officials may have received [rewards] for helping secure decisions to remove such people from the UK is very worrying indeed."

The disclosures about the incentives come after Duncan Lewis wrote to the Home Office protesting at long delays in handling the some asylum-seeker cases. The firm believes some cases with strong grounds for appeal are being withdrawn by the Home Office on the day of the tribunal because officials fear they will lose and risk failing to meet their target of winning 70% of cases. The applicant then has to wait even longer for the Home Office to make a fresh decision.

This allegation is rejected by the Home Office, which said officers did not prioritise cases depending on the likelihood of success and added that "any decision to withdraw a case has to be approved at a more senior level".