The leaders of the BBC today admitted that the lewd phone messages broadcast by Jonathan Ross and Russell Brand on Radio 2 last month were a grave lapse of standards but said that the corporation had done its best to put things right.

Facing questions by MPs in a Culture, Media and Sport Committee hearing at the House of Commons, director-general Mark Thompson said: "I am very aware that this was a very serious editorial lapse. There were errors in judgment."

Sir Michael Lyons, the BBC chairman, who was also questioned by the committee, said that the corporation had "crossed a boundary" by broadcasting the messages left on actor Andrew Sachs’s answerphone on Brand’s Radio 2 show.

But under uncomfortable questioning Sir Michael and Mr Thompson insisted that the corporation had acted promptly and properly in the aftermath of the scandal, which prompted more than 40,000 people to complain to the BBC.

"There was no lack of speed. This is something I do not accept," said Sir Michael, when MPs accused the BBC of lamentable slowness.

"I refute and reject any allegations there were more actions the trust should have taken. The trust is doing its job of holding the executive to account."

The Culture, Media and Sport Committee decided to focus on the controversial broadcast as it held the second hearing in its inquiry into the BBC's commercial operations.

On October 18 Brand and Ross could be heard on air leaving a string of offensive messages, claiming that Brand had slept with Sachs' granddaughter Georgina, after Sachs failed to answer for a pre-arranged phone interview.

Lesley Douglas, the head of Radio 2, quit on October 30 in the wake of the scandal, followed by David Barber, the Radio 2 head of specialist music and compliance. It is thought Barber’s job would have involved checking that content complied with BBC guidelines.

Andy Burnham, the Culture Secretary, has said that the calls to Sachs were a "serious lapse of broadcasting standards" and that the BBC management was "too slow" in recognising the seriousness of the situation.

The controversy is still under internal investigation, and the BBC Trust is due to discuss the furore, which saw Brand quit and Ross suspended, when it meets later this week. It has requested a final, written report on the matter from the director-general.

The MPs criticised Mr Thompson and Sir Michael for failing to fire the two presenters for "gross misconduct", but Sir Michael said: "Before you bandy around words like gross misconduct, the BBC has a duty of care.

"The BBC looks at the evidence before making its decisions. I think that’s the sign of a healthy organisation."

Mr Thompson said the BBC had made improvements over the past five years, pleading: "You cannot expect that sometimes we will not get it wrong. This is a very uncharacteristic."

Responding to accusations of systematic failures, Sir Michael admitted there were lessons to be learned, but added: "It is in the nature of the BBC that it takes risks."

Mr Thompson denied that Ross's £6 million a year salary was too high. "I think, if the BBC has to have top talent, you have to accept that. Even when you grow your own talent, people are on the phone," he said.

The MPs are holding an inquiry into the benefits, opportunities and risks of the BBC's profit-making activities. The future of its commercial arm, BBC Worldwide, and of other BBC commercial subsidiaries are all under scrutiny, and how the money earned by the BBC is invested.

Mr Thompson defended BBC Worldwide, saying that it had repeatedly been voted the distributor of choice by the industry. "The BBC has been able to make programmes it would not have been able to do without BBC Worldwide," he said.

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