It agreed to sponsor the food and drink for 450 guests at a high-profile event in Brighton to celebrate Labour's 100th conference only after receiving assurances that the prime minister would attend.

Hosted by the cabinet minister Charles Clarke, appointed party chairman by Mr Blair after the election, the invitation-only soiree, on the eve of the leader's keynote speech, is one of the most controversial commercial "opportunities" offered by Millbank to raise upwards of £4m during the week.

The US corporation, a target of anti-globalisation protesters who plan to demonstrate outside the conference when it opens on September 30, is anxious to receive the implied endorsement of Mr Blair and other ministers.

Executives initially intended to finance a football stunt within the conference security complex, but switched when they found that Mr Blair was unable to make the kickabout.

McDonald's, advised by lobbyists Weber Shandwick, said a director would "say a few words" at the reception. A corporate video crew is expected to film the gathering. The company also raised the possibility of the prime minister wearing a McDonald's hat, according to a lobbyist with knowledge of the discussions, but Millbank ruled that out as unacceptable.

The arrangement last night prompted unease within the party's own ranks amid mounting criticism of the Labour leadership's use of the conference as a money-spinning convention.

The Labour MP John McDonnell, a critic of transnational corporations such as McDonald's, said: "It turns my stomach, I don't know how low we can sink. It just shows how out of touch the Labour leadership is with the rank and file of the movement."

John Edmonds, the head of the GMB who fears unions are being sidelined in the party, said: "I hope this is not the first step towards renaming us the McLabour party. If a company is prepared to pay £15,000 for the privilege of a glass of warm white wine with the prime minister, then it can afford to pay decent wages."

A Millbank brochure sent to companies, a copy of which has been obtained by the Guardian, lists 30 "sponsorship opportunities". It invites firms to sponsor official fringe meetings involving ministerial teams, including one on crime with the home secretary, David Blunkett, and another on education with Estelle Morris. A company adviser approached by Millbank disclosed that the party HQ was seeking £10,000 to attach a firm's name and logo to the question and answer sessions.

Delegates were described as a "captive audience", with the conference's ambulance service, relaxation zone, WAP phone service, video screens, recycling bins, gala dinner flower arrangements and a meeting on public services all up for grabs.

McDonald's said that it had sponsored other Labour receptions in the past two years and would also stage events at the Tory and Lib Dem conferences. "It's an opportunity to talk about our community-based activities to a big group in one go," a spokeswoman said.

The Labour party also insisted that the company's involvement was "not unusual. An official said all parties sought sponsorship without conceding influence.

A McDonald's franchise was fined £12,400 last month after admitting 20 offences of illegally employing children aged 15 and 16 at two Surrey restaurants. The fines, among the highest imposed on for breaking laws on child working conditions, were levied after a court heard that one pupil had worked a 16-hour Saturday shift and another until 2am on a schoolday.

For sale: access to captive audience

Examples of New Labour sponsorship "opportunities" at the party's annual conference:

Chair's reception "NEW! 8pm to late. A major reception celebrating the 100th Labour conference. The event will be hosted by the party chair and general secretary and will be attended by up to 450 guests in the Hilton Metropole's Balmoral Suite" Price £15,000

Policy fringe: education "A question and answer-style event with the education team" £10,000

Ambulance service "NEW! Enjoy a positive profile through an association with the vital emergency service provided by the local ambulance service" £5,000

Conference dinner: flowers "Branding opportunity on every flower arrangement provided for the 700 guests" £3,500

Relaxation zone "NEW! This increasingly popular relaxation therapy facility is free for all visitors and delegates. A unique branding opportunity to a captive audience" £7,500

Leader's speech video screen "A unique promotional opportunity for the special video screen provided for hundreds of delegates to watch leader's speech" £4,000

Costs are based on the estimates of lobbyists involved in negotiations with Millbank  
 
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