The SNP is this weekend calling on the government to confirm that the proposed cull of Scottish MPs at Westminster will take place before the next general election.

Under the terms of the Scotland Act the number of Scots MPs should drop from 72 to 59 to mirror the average population size of constituencies in the rest of the UK.

Following a consultation, however, the number of MSPs in the Holyrood parliament will remain the same - a move which will break the "coterminous" boundaries between MPs and MSPs.

The Commons will soon debate government legislation which amends the Scotland Act to prevent Holyrood being forced to follow the Westminster reduction.

Ahead of the second reading of the Scottish Parliament (Constituencies) Bill the nationalists are demanding a cast-iron guarantee that the reduction will go ahead at Westminster.

In a letter to Scottish secretary Alistair Darling the SNP's Pete Wishart called for clarification from the government.

"You will know that the second reading of the Scottish Parliament (Constituencies) Bill is now timetabled for Monday February 9," he said.

"I am expecting that prior to this debate you will make a definite statement indicating that you are going to accept the recommendations of the Boundary Commission, and that the Westminster parliamentary constituencies will definitely be reduced from 72 to 59 in time for the next general election.

"I believe it is imperative that you do this, otherwise the debate on February 9 will be purely hypothetical and based merely on the assumption that there will be a reduction in the number of Scottish Members.

"I am sure you will agree with me that to conduct the debate without an unequivocal statement that the number of Scottish MPs will indeed be 59 at the general election would be very unsatisfactory."

A spokesman for the Scotland Office said the government's commitment to a reduction in the number of MPs still stood.

"It is still our intention to go ahead with the reduction of the number of MPs at Westminster," she said. "There has been no change."