The move comes as lobbying companies prepare to cash in on devolution. Many big London-based firms now have offices in Edinburgh and Cardiff, and several indigenous companies have also sprung up.
Several Edinburgh law firms have set up lobbying arms, hoping to make money by drafting amendments to legislation on behalf of commercial clients.
Lord Fraser of Carmyllie, a former Conservative deputy leader in the House of Lords and a Scottish Office and industry minister in the last government, is chairman of the newly formed Holyrood Strategy (Scotland). The Liberal Democrats' federal treasurer, Denis Robertson Sullivan, is managing director of PS Public Affairs.
But while Westminster MPs are allowed to take on consultancies with lobbying firms, their Scottish counterparts will be told that such positions are "inappropriate". In Wales therules will reflect those at Westminster, where such interests must merely be registered.
The conclusions of a working group on rules for Scottish Parliament members, expected to be approved by the Parliament, says MSPs should be barred from taking fees not only from commercial lobbyists but also from PR firms or lawyers who are pressing for particular policies.
The code of conduct "should be a powerful tool in discouraging MSPs and lobbyists from participating in irregular activities", the working group said.
"We consider that it is inappropriate for any MSP to receive payment in cash or kind from any firm engaged in lobbying on a commercial basis."
Some lobbyists and politicians believe the group should have introduced compulsory registration for lobbying firms. They would then have to sign up to rules likely to bar them from offering cash to politicians for favours or from appointing politicians to their boards, as the existing voluntary code does.
Robbie MacDuff, managing director of Strategy in Scotland, the Edinburgh branch of the English lobbying firm Westminster Strategy, said it was "naive" to think lobbyists could regulate themselves. "I don't think Scotland is so special that it will work here when it doesn't work in London," he said. "In an industry where people are chums, there are too many temptations."
Mr MacDuff questioned whether members of the House of Lords should be allowed to work for Scottish lobbying firms. In England, peers must declare their lobbying interests.
Lord Sempill, a hereditary peer who runs Holyrood Strategy (Scotland), chaired by Lord Fraser of Carmyllie, defended his right to be in the business. The peers were told that they did not need to register their interest in the House of Lords' register of members' interests.
Lord Sempill is standing as a Conservative for the Scottish Parliament but does not expect to win. "Quite frankly, I have to have a career. I think we clearly understand amongst ourselves in the fraternity that there is a conflict of interests in holding an active role in a public affairs company and at the same time being part of the political process," he said.