The Liberal Democrats have begun drawing up conditions for a coalition deal in the event of a hung Parliament.

In an interview with The Times to mark his first anniversary as party leader, Sir Menzies Campbell said that the price of putting Gordon Brown in Number 10 would be a firm commitment to the introduction of proportional representation at Westminster.

The success of the Liberal Democrats in Scotland, where they secured proportional representation for local council elections, has made the party “more hard edged” about demanding its introduction for the rest of the country, he said.

“The party has moved. The party’s view is pretty clear about PR. If you look at Scotland, the Scottish Parliament, local government and European elections, they were all by proportional representation. The only one that isn’t is Westminster,” he said.

The Liberal Democrat leader confirmed that he had asked Paul Burstow, his chief whip, to report on how coalition talks are conducted. “You would be very surprised if I wasn’t getting assistance from any source that I thought helpful,” he said. The report will examine the conventions surrounding coalition discussions, last used in February 1974.

Sir Menzies attempted to play down the move, saying that it was part of wider election preparations, and insisted that he did not want the party to become distracted by theoretical discussions. “I am by no means convinced of the statistical probability of a hung Parliament,” he said. “I think the one thing you cannot campaign for is a hung Parliament, you have to campaign for your own policies and your own values.”

Speaking on the eve of the party’s spring conference, which starts in Harrogate on Friday, Sir Menzies would not rule out going into coalition with the Conservative Party. In the event of a hung Parliament, “there will be fine judgments to be made”, he said.

But the Liberal Democrat leader also indicated that there was little personal chemistry between him and the Conservative leader. He contrasted his relationship with Mr Blair, who “has very good manners”, and Gordon Brown, who “was around forever”, with his more formal relationship with David Cameron. “Curiously, I know one or two people who know him, but [we don’t talk much] apart from being polite as we wait to go out to the Cenotaph or sit in St Paul’s together.”

In the past year Sir Menzies has launched a number of personal attacks on the Conservative leader, calling on him to apologise for supporting the Iraq war and branding his party a glorified advertising agency during conference.

Yesterday he made a fresh attack: “I noticed that Mr Cameron was saying we should let people stand up at Premiership football matches. You slightly wonder, if someone sent him a petition saying that the Earth is flat, would he say that this is a proposition that required careful scientific analysis.”

Although Sir Menzies has criticised some aspects of the Chancellor’s work, he refrained from making any withering statements about the Chancellor. Asked how a Brown premiership would differ from Mr Blair’s, he replied: “I don’t think we know. I think the economy will be the same, the authoritarianism will be the same, the targets will be the same. That’s the way he is. Expect rabbits out of hats.”

Would the Liberal Democrats challenge any major surprises produced by Mr Brown on becoming leader? “That depends on what colour the rabbit is,” he replied.

However, Sir Menzies sounded a warning for Mr Blair after he leaves office, to avoid anything too “evangelical” and to keep out of Mr Brown’s way. “If he is going to do something with a Blair Foundation, I think he would be advised to do something with a social purpose, rather than political.”

Sir Menzies defended his decision not to call for the resignation of any government ministers since taking over, apart from Mr Blair, who he has said should stand down now. “Well there’s a kind of ritual about that and I hope that I’m not a ritualistic politician,” he said.

He also confirmed that his wife, Elspeth, plays an important role in his political life. “We’ve always done it together. Right from the first time I took her to Greenock in 1974 to the shipyard meetings with loud hailers.”

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