Tony Blair has come under pressure over the government's failure to "get a grip" on a continuing crisis at the Child Support Agency.

During exchanges in the Commons, Michael Howard and Charles Kennedy urged the prime minister to tackle problems that have left some families without the payments to which they are entitled.

The criticism came after work and pensions secretary Alan Johnson announced the resignation of CSA boss Doug Smith.

Reports suggest the CSA has reached breaking point, with only 43 per cent of cash for single parents and children being collected.

A year ago the government claimed the problems would be solved by a simpler maintenance calculation and a £456 million computer system.

But describing the CSA as "in crisis", the Tory chief urged the prime minister to "get a grip" on the continuing difficulties.

Howard told MPs that continuing technology problems meant the CSA had failed to collect around £750 million, much of which was owed to some of the poorest families in Britain.

"These people desperately needed that money to help bring up their children," he said. "They have been denied it by the failures of this government."

Blair admitted that the IT problems should be "sorted out" because they had caused many problems.

But he said that the government had introduced a new simplified system for assessing payments owed by absent parents.

"I am very happy to take lessons from people who have found they have not got the service they need under the new system," said the prime minister.

"What I will not do is take lessons from the Conservative Party that introduced a child support system that was a by-word for chaos and unfairness."

'Scrap the CSA'

During the exchanges the Liberal Democrat leader called for the CSA to be "scrapped", with its functions transferred to the Inland Revenue.

"As well as the computerisation problems there is a problem that the government has been changing the specifications," Charles Kennedy said.

He added that the problems stemmed from "unrealistic targets" that are a "central problem with this government".

Blair said he would look at the idea but said that it was unlikely to be introduced.

The prime minister said the new CSA leadership would have to get to grips with the problem, adding that the IT system would eventually be "simpler".

"It seems to me highly unlikely that transferring this to the Inland Revenue will cause anything other than consternation in the Inland Revenue and others," he told MPs.

Resignation move

The news of Smith's resignation as CSA chief was announced during evidence to the work and pensions committee on Wednesday.

Johnson told the committee that the departing official "has decided that now is the time to stand aside".

"He believes we have reached a natural breakpoint at which to hand over the reins," added the pensions secretary. "I would like to publicly thank him for his work in what has been a terribly difficult time."

Committee chairman Sir Archy Kirkwood attacked the agency for causing "unnecessary suffering" to single parents, partly because of computer problems.

Smith told the committee he was not leaving because he had "failed to deliver".

"I'm leaving the CSA because I indicated when I took this job I expected to do it for between three to four years," he said.

"I think I and my senior management team have done a good job over the last year to mask the worst impacts of this IT system from the people who really count in this, who are our clients who are looking for money to support families in adverse circumstances."