Plaid Cymru has said the Welsh assembly should take legal action to prevent the Home Office pressing ahead with its policy of police force mergers.

Home secretary Charles Clarke has said the four Welsh forces should merge into a single body.

Ministers in Westminster believe the merger would deliver efficiency savings while also improving police capability for dealing with serious crimes.

But opposition parties in the assembly are critical of the plans, and could move to block them.

Under Section 38 of the Government of Wales Act 1998, the assembly has the power apply for judicial review of various decisions.

And Plaid Cymru social justice spokesman Leanne Wood has called for legal advice to be taken on halting the police mergers by applying for a judicial review of the Home Office's decision.

She said that such an application by the assembly would be the first such legal action taken by one crown body against another.

"Plaid Cymru believes that the way Labour in London is merging our police forces is irrational, ill-timed and insulting," said Wood.

"The assembly should take legal action to stop this restructuring, after all the safety of our communities is at stake.

"It would be a legal first, but Wales' last chance for a common sense approach.

"This whole episode shows that criminal justice policy needs to be devolved to the assembly.

"Charles Clarke has shown himself incapable of running our police forces."

Welsh assembly Liberal Democrat leader Mike German said he had written to the chairs of the Welsh police authorities asking them to consider launching a judicial review of the Home Office.

"The home secretary has done his best to ignore the objections of people in Wales and the concerns of senior police officials," German said.

"Starting a judicial review process is a means of forcing the Home Office to review its decision within a proper and sensible timescale.

"I'm not encouraging the police authorities to become involved in a protracted legal action against the Home Office.

"Instead, the 'letter before claim' will formally express concerns, force the home secretary to address the issues, and establish whether litigation can be avoided."