'I don't think it's any of your business,' he told Helen Steel and Dave Morris in the High Court.
Mr Justice Bell ruled their question out of order 'for the moment', although he said he would keep an open mind.
Ms Steel and Mr Morris argue that McDonald's is one of many multinational corporations that pay shopfloor workers as little as possible while making big profits and awarding top management large salaries.
McDonald's is arguing that its wages of around pounds 3 an hour are highly competitive in Britain's fast food sector.
Mr Preston shrugged off comparisons between his salary, which he said was typical for a senior executive in a large company, and the wages paid to shopfloor staff.
World-wide, the corporation made a profit last year of just over dollars 1bn. Cross-examining Mr Preston, Ms Steel suggested some of that profit could be used to raise wages. 'I suppose they could be paid higher,' Mr Preston said.
He added that it was his decision to pursue the libel action over an anti-McDonald's leaflet. The leaflet was published by London Greenpeace, a group with ecological and anarchist sympathies which is no relation to Greenpeace International and pre-dates it.
The case will last some three months and is likely to cost more than pounds 1m, with McDonald's having to foot the bill because the defendants have no substantial assets. They have no legal aid and are defending themselves, claiming that although they did not write or distribute the leaflet its contents are true or fair comment. The leaflet said McDonald's food is unhealthy and unsafe and that its production contributes to rainforest destruction and Third World hunger.
The case continues next week.Reuse content