Jim Wallace was speaking as opinion polls suggested no party will have the overall majority needed to form an unassailable administration at Holyrood.

BBC Scotland Correspondent Andrew Cassell reports

Mr Wallace, who was visiting the Irn Bru factory in Cumbernauld with Liberal Democrat leader Paddy Ashdown, said unless Liberal Democrat ministers were appointed in the Scottish Parliament executive his party would not be prepared to sup with others in a coalition.

He said the party is not interested in a looser arrangement hinted at by Labour leaders.

Jim Wallace says Lib Dems must form part of executive

"We're not going to put ourselves in a position where we are supporting a government position from the opposition benches and getting all the share of the blame when things go wrong and none of the credit when things go right," said Mr Wallace.

"If it comes that no agreement can be struck we are prepared to sit on the opposition benches."

Labour sought to brush aside talk of coalition as Scottish Secretary Donald Dewar and Deputy Prime Minister John Prescott toured Glasgow.

BBC Scotland political correspondent Kenny MacIntyre reports:

Mr Dewar said: "I don't know what the Liberal Democrats are saying, they haven't been saying it to me and I haven't been talking to them.

"We've got one aim and that is victory on Thursday."

However, in a newspaper interview on Tuesday, Mr Dewar said political stability in the new parliament was unlikely to come from a minority administration.

"I think at the end of the day, stability is a great prize," he added.

Elsewhere on the campaign trail, Manchester United manager Alex Ferguson joined Labour's elections campaign, urging the electorate to "make it a hat trick" for Labour on 6 May.

Tory leader David McLetchie in law and order message

He called on Scots to cast all three votes for Labour in the parliamentary and local council elections.

His "hat trick"message was unveiled on a giant poster on an ad trailer in Glasgow's Sauchiehall Street.

The Scottish National Party, meanwhile, was celebrating after an opinion poll in The Express newspaper suggested the party was within four points of Labour on the second vote.

The NOP poll indicated SNP support has risen by three points to 31% on the first vote while Labour support has fallen by three points to 46% on a previous poll conducted on 19 April.

On the second vote, Labour support has fallen by six points to 44% and the SNP now has a 32% share of the vote. This would give Labour 56 seats and the SNP 43.

SNP leader Alex Salmond also dismissed any talk of coalition in the closing stages of the campaign.

He said: "If Labour and Liberal politicians want to carve up ministerial seats that is a matter for them.

"The dialogue I am having at the present moment is with the people of Scotland."

Second vote

The opinion poll suggested Liberal Democrat support has fallen three points to 8% on the first vote and remained static at 10% on the second vote, translating into 11 seats.

Tory support, however, has risen from 9% in both first and second votes to 12% in the first vote and 14% in the second - 17 seats.

Party leader David McLetchie said he views coalition government as unprincipled.

He said: "I don't think there is a need for coalition politics, I think there is a need for honest politics in the Scottish Parliament and not these shabby, backstairs deals.