Asylum seekers with strong cases are being pushed to the back of the Home Office queue so that officials can reject at least 60% of applications, according to lawyers.

Those with clients at the appeal stage say that it has become common Home Office practice to withdraw powerful cases at the last minute to protect the 60% target, described internally as the "win rate". They say the alleged tactic is adding to a backlog of half a million unresolved cases and causing suffering to those who have a right to be here but who are stranded in limbo, sometimes for years.

The latest controversy follows criticism of the Home Office for launching a campaign using vans carrying billboard slogans urging illegal migrants to go home or face arrest. It was announced last week that the controversial London pilot scheme would not be extended across the country.

Home Office officials dealing with asylum cases are expected to secure a rejection in the majority of cases. Those who fall short are given "extra support" to help them meet the 60% target.

Immigration barrister Colin Yeo said that he sometimes comes across two or three cases a week where the Home Office has withdrawn from strong cases just before they are due to be heard.

"This is simply a way of the Home Office trying to hit their targets. We deal with cases that we are fairly confident we are going to win, only to see them disappear down the back of a sofa at the Home Office. It's short-term thinking that is contributing to the long-term problem."

James Packer, head of public law at Duncan Lewis solicitors, said: "The whole thing is a scandal. Very often decisions are withdrawn at the last moment because it is apparent that if the appeal were heard it would succeed. The tribunal has no power to prevent this, and a case can sit there for another year or more before a fresh decision is made, causing real suffering and preventing people from getting on with their lives. That's the mischief they create to defend their 'win rates."

The MP Keith Vaz said he was writing to the immigration minister Mark Harper to ask him to explain what guidance was issued to presenting officers to protect the most vulnerable.

He told the Observer: "It is completely unacceptable that the Home Office appear to be deliberately adding to the backlogs to meet short-term targets. Many of those who have sought asylum in the UK are extremely vulnerable. The practice by the Home Office of rewarding those with a high win rate seems callous."

A Home Office spokesman confirmed that officers were expected to meet targets of having at least 60% of cases refused at court, adding that when decisions at tribunal were withdrawn a senior officer's approval was needed.

The spokesman added: "The success of our officers in upholding asylum decisions is only one of a range of criteria we use to monitor staff performance. All our staff are expected to meet appropriate professional standards."

■ Leading Tory peers have accused the government of failing to provide evidence that people from other European Union countries are using the UK for "benefit tourism" at huge cost to the British taxpayer.

Lord Boswell, who chairs the European Union committee in the House of Lords, said in a letter to immigration minister Mark Harper, leaked to the Observer, that he was "concerned about the lack of data" that ministers had supplied to support their claims.

Boswell said figures provided to the committee in response to a previous call for evidence were "not particularly illuminating", and argued that the committee had been given clear figures from other sources that suggested the opposite – that EU migrants pay more into the economy than they take out.

The European commission is also growing increasingly frustrated with the UK government for claiming that "benefit tourism" is a massive problem while not supplying any verifiable data to support the claim.

EU officials believe the government is trying to create an impression that there is a problem for political reasons, so that David Cameron can claim to be dealing with it when he comes to renegotiate the terms of the UK's EU membership.