They allege the minister has moved �20m from two other water authorities but has also given a further �20m of government cash.

Jack McConnell will be quizzed

The party's environment spokesman, Murray Tosh, said the parliament must be told where the money has come from.

The move by the Tories comes in the wake of calls for a full inquiry into why North of Scotland Water Authority (NOSWA) plans massive increase in charges next year.

The Scottish Executive has already asked NOSWA to carry out an internal investigation.

The authority serves just over a million consumers in an area covering 60% of Scotland.

Extra charges

It has reviewed its investment programme and said a substantial increase in charges was required, but has refused to comment on reports consumers could pay as much as �55 a year extra.

The reported increase has been much greater than that expected in Scotland's other two water authorities.

Some MPs and MSPs in the area have called for a public inquiry.