Mr McConnell (left) came under fire from the Socialist leader (right)

Presiding Officer George Reid launched an investigation after Mr Sheridan accused Jack McConnell of "lying" to parliament over the war in Iraq.

The clash came a week after Mr Sheridan made similar claims over comments Mr McConnell made on the SSP drugs policy.

Mr McConnell considers Mr Sheridan is lowering standards in the chamber.

'Guerilla warfare'

Mr Sheridan made his comments during first minister's question time on Thursday.

He asked Mr McConnell if he would apologise for misleading parliament over the Iraq war.

"Or is it the case that you think lying in parliament is now an acceptable practice as you did last week in relation to the SSP's drug policy and, of course, you did in relation to trying to justify the illegal and unnecessary invasion of Iraq?" he said.

Parliament Minister Patricia Ferguson called on Mr Reid to step in to end the row.

She said: "This is now becoming a sustained period of conduct which frankly does not befit members of this parliament."

Mr Reid said his predecessor Sir David Steel had ruled in 2000 that the use of the word "liar" by MSPs would not be tolerated.

But he said he would investigate the row in a bid to defuse the matter.

He said: "I don't want a period of sustained guerilla warfare in this house, it does none of us any good."