The influence of lobbyists in the House of Lords will come under increasing scrutiny as official inquiries continue into the cash-for-influence row engulfing four Labour peers.

Lords Leader Baroness Royall has pledged to toughen the upper chamber's anti-sleaze rules as she launched two parliamentary probes into claims Labour members were ready to change legislation in return for money.

Lord Truscott, Lord Moonie, Lord Taylor of Blackburn and Lord Snape are accused of entering into negotiations, involving fees of up to £120,000, with Sunday Times reporters posing as lobbyists for a foreign firm.

All four strenuously deny any wrongdoing but they face the possibility of a police probe after the Liberal Democrats filed a complaint with Scotland Yard.

And Lord Moonie, a former defence minister, also faces separate questions over his decision to give a Parliamentary pass to someone previously stripped of such access because of his arms industry lobbying links.

The four peers will give evidence to the House of Lords interests sub-committee privately this week, while Lady Royall has been holding talks with them in her role as Labour's leader in the second chamber.

Answering an urgent question on the issue, Lady Royall said the sub-committee on interests had already started investigating and that the chairman of the Privileges Committee, Lord Brabazon of Tara, was carrying out a wider review of punishments, raising suggestions peers caught breaking rules could face expulsion or suspension.

Currently peers found to have broken conduct rules can only be ordered to apologise to the House.