MPs have been given the go-ahead to continue using a controversial procedure to block the questioning of witnesses by Commons committees.

But they have been warned only to use the 60-year-old rule in "extreme circumstances" or risk losing the privilege altogether.

The row erupted during two recent meetings of the Foreign Affairs select committee which was taking evidence on the arms-for-Africa affair.

Labour member Ernie Ross pitched the proceedings into chaos when he invoked the little-known "object" rule to stop MPs cross examining both Foreign Secretary Robin Cook and top diplomat Sir John Kerr.

The move led to angry Tory complaints that the MP was deliberately trying to gag the committee and stop it carrying out its legitimate duties.

Question withdrawn

On both occasions, the committee was forced to go into private session to decide whether the questions should be allowed.

In the event, one question was withdrawn but the second was allowed to be asked again.

But committee members, including some Labour MPs, claimed the move meant any member could invoke the rule to effectively kill off any line of questioning he did not like.

In a bid to avert a serious constitutional clash, the matter was referred to the Commons Procedure Committee for a final ruling.

That committee has now decided that the rule may sometimes be useful to protect witnesses from being forced to answer unfair questions.

But it warns that, if the rule is used regularly in the future, it will reconsider the issue.

"There has been no ready recourse to the procedure in the past. If there is in the future, we undertake to look at this matter again.