May says agency will be split in two with separate immigration and law enforcement services under direct control of ministers

The troubled UK Border Agency is to be abolished and brought back within the Home Office, the home secretary, Theresa May has announced.

She told MPs that she will also split the "closed and secretive" agency into an immigration and visa service and a separate law enforcement command while bringing it back under the direct control of ministers.

May first split off the UK border force from UKBA 12 months ago in the wake of the Brodie Clark affair.

The home secretary said in an unscheduled Commons statement that UKBA was "a troubled organisation … its performance was not good enough". She identified four main problems: its size, its lack of transparency, its IT systems and its policy and legal framework.

She said that the immigration agency has been such a "troubled organisation" for so many years that it will take many more years to clear the backlogs, which now top more than 310,000 cases, and fix the system.

"UKBA was given agency status in order to keep its work at an arm's length from ministers. That was wrong," said May. "It created a closed, secretive and defensive culture. The new entities will not have agency status and will sit in the Home Office, reporting to ministers."

Keith Vaz, the chairman of the Commons home affairs select committee, which accused the former UKBA head, Lin Homer, of "catastrophic leadership failure" while she was in charge, congratulated May for "delivering the lethal injection" to the organisation.

"She has done the right thing in putting UKBA out of its misery. The organisation is not fit for purpose. However, this cannot be an excuse not to clear the backlogs, which stand at a third of a million cases. Ministers are now in the front line."

The home secretary said that sheer size of the £1bn-a-year organisation meant it had conflicting cultures and all too often focused on the crisis in hand at the expense of other important work.

She also announced her intention to bring in legislation this autumn to make it far easier to carry out asylum and immigration deportations. As the Guardian disclosed on Saturday almost half the forced removals attempted by UKBA have to be cancelled following successful legal challenges.

May told MPs: "The agency is often caught up a vicious cycle of complex law and poor enforcement of its own policies, which makes it harder to remove people who are here illegally," she said.

The agency was set up in April 2008 in the aftermath of John Reid's infamous declaration in July 2006 that the old Home Office immigration and nationality directorate was "not fit for purpose".

Its abolition follows a scathing report from the Commons home affairs committee which criticised the former UKBA head, Lin Homer, of a "catastrophic leadership failure" during her period in charge from 2008 to 2011, and revealed that the backlog of immigration and asylum cases had now reached more than 310,000. Vaz, said it would take 24 years at the current rate of progress to clear the backlogs. He said his committee had been repeatedly misled about the backlogs.

"It appears more like the scene of a Whitehall farce then a government agency operating in the 21st century," he said.

The shadow home secretary, Yvette Cooper, however, claimed that UKBA performance had got worse not better under May's watch.

"The home secretary is right to say there are problems at UKBA and it has had a series of problems over many years," said Cooper.

"We would have some sympathy with your proposals but the trouble with the proposals is you are simply refusing to recognise problems around enforcement and effectiveness at UKBA have got worse and not better on your watch.

"Enforcement has got worse, delays have got worse, 50% fewer people are being refused entry at ports and borders and you said the number of illegal immigrants removed does not keep up with the number here illegally – that's because you're letting rather more of them in."