They will be urged to pay pounds 1,000 a year in subscriptions to receive detailed papers from the party's Millbank policy unit, reports from the national policy forum and all-party press releases announcing policy decisions.
The deal does not involve direct access to the policy-making bodies, but it will help the lobbyists prepare strategies for their clients by getting an early sight of party policy thinking.
A Labour Party spokesman denied that the seminar on 31 March would be a fund-raising event. "We are inviting public affairs companies and corporate affairs executives. It is about explaining our policy process and the `Partnership in Power' document that was adopted. We do look for sponsorship, but it is not directly to do with that."
The lobbyists will get papers being discussed by the national policy forum which is replacing the annual party conference as the main policy making body in the party.
"We explain how the policy process works and we can show them why it would be in their interests to pay pounds 1,000 a year to receive the reports to our policy forums. We will be telling the firms the material that comes from the national policy forum will be available to them if they subscribe," said the spokesman.
A spokesman for William Hague, the Tory party leader, said: "These lobbyists will not be going for fun. They want access and influence for their clients."
Tony Blair was elected partly on a pledge to clean up British politics after the disclosures of Tory sleaze, and a number of lobbyists have gone out of their way to clean up the reputation of their business.
The disclosure that Bernie Ecclestone, head of Formula One motor racing, had donated pounds 1m before lobbying the Prime Minister to stop a ban on tobacco advertising on the sport opened the Government to the charge of Labour accepting large sums of money for influence. It repaid the money, but the charge was reopened by the Tories following a report in Sunday Business that Lord Sainsbury had paid pounds 2m to the party, twice the sum earlier reported, through his solicitors.
Lord Sainsbury, who was awarded a life peerage after the election by Mr Blair, is a member of the family which owns the supermarket chain that lobbied with Tesco and Safeway against planning controls on out-of-town supermarkets.
Ministers are adamant that no one can buy influence with the Government, but the Tories are planning to raise the reports in the Commons this week.
A Tory party spokesman said: "What is of great concern is the suggestion that yet another large donation to the Labour Party has bought a policy U-turn."
The Tories will also be asking questions about the seminar for lobbyists. Invitations which are going out to the public relations companies offer policy forum papers, a monthly review of government policy by the policy unit within the Labour Party.
The speakers at the seminar include Matthew Taylor, assistant general secretary of the party and Hazel Blears, the new MP for Salford, and a member of the national policy forum. Those attending will also be able to meet members of the party's internal policy unit.
The seminar is highly-sensitive for the party, which is anxious to avoid being accused of giving greater access to lobbyists than their own members to the production of party policy.
Some traditional Labour Party supporters were highly suspicious of the establishment of the national policy forum and the threat it represented to the power of the left-wing activists to gain a platform through the constituency parties at the annual party conference.
Prescott orders inquiry, page 8