The government is to come under intense new pressure over the "cash-for-access" row.

The opposition is seeking ways of raising the matter in the Commons, either through an emergency debate or questions, in a concerted bid to keep ministers on the spot over the issue.

Shadow Chancellor Francis Maude has written to Chancellor Gordon Brown after claims his most senior advisers had leaked budget secrets.

He said the claims had raised a "serious question mark over the Treasury". The allegation has been fiercely denied.

And backbenchers are pressing for tough new action against the lobbying firms at the centre of the allegations.

Home Secretary Jack Straw defended the government, claiming the allegations were merely annecdotes.

Quarantine period

But he went on to suggest Lord Neill's anti-sleaze committee should be asked to consider the benefits of a "quarantine period" which would ban former party workers moving to lobbying firms for a set period after their party won power.

Draper denies wrongdoing

And Labour backbencher Gordon Prentice demanded the former Labour aide at the centre of the claims, Derek Draper, should be kicked out of the party.

The row surrounds newspaper allegations that ex-Labour advisers have colluded with government insiders to gain privileged access to ministers and key policy documents for clients.

The lobbyists and aides named have all denied doing anything improper and the government has stressed none of the reports contained any allegations against ministers.

Mr Draper has admitted being "boastful" about his contacts with the government, but rejected suggestions of improper behaviour.

He has already been sacked by his firm and the newspaper he used to write a column for. Fresh allegations over the weekend led to more denials from Minister Without Portfolio Peter Mandelson, who used to employ Mr Draper.

Lunch with adviser

Mandelson at centre of claims

It was reported the lobbyist had regularly received faxes from Mr Mandelson's office - but both men said nothing confidential was ever passed on.

It was also said that Mr Draper had lunched the chancellor's adviser, Ed Balls.

In another statement, Mr Draper insisted he had done nothing secret or improper.

"To be accused of having lunch with someone at the Savoy, which is a public place, and received faxes which nobody even suggests were sensitive or confidential is quite extraordinary," he said.

However, there is mounting pressure for new rules to be used to control the activities of lobbyists.

The Association of Professional Political Consultants has demanded that two of the groups named in the allegations - GPC Market Access and GJW - send directors to an emergency meeting on Tuesday to explain themselves.