London mayor Boris Johnson has reacted cautiously to a call by the head of the Metropolitan Police for the force to be allowed to recruit one ethnic minority officer for every white officer they take on.

Commissioner Sir Bernard Hogan-Howe has called for a change in the law to allow positive discrimination over a five year period amid accusations that the predominantly white force is still "institutionally racist".

The proposal is based on the model used in Northern Ireland where a temporary change in the law allowed the newly formed Police Service of Northern Ireland to take on one Catholic for every Protestant it recruited.

Mr Johnson said that while he wanted to see a force that "looks like London", any move towards positive discrimination would have to be examined very at carefully.

"At the moment I'm reflecting on it. I think it is something we need to consider. It's not obviously part of our traditions here in England. There are all sorts of ramifications that you have got to think through," he told LBC radio.

"This would be a big change. I think we'll need to debate it, we'll have to have a proper conversation with London. I imagine we'll have a consultation before we do such a thing. I don't think this is something that I'd just want to enter into at the drop of a hat.

"The potential of the idea is that it does seem to have worked in Northern Ireland. The downside is ... that whenever you have a positive discrimination move you are inevitably going to set up certain problems. You just need to reflect."

He warned that it could undermine the position of ethnic minority officers if it was thought that they were only appointed because of their ethnicity rather than on merit.

"You would be undermining his or her own confidence in his or her own success and achievement," he said.

Mr Johnson strongly defended Sir Bernard after he received a rough ride from MPs on the Commons Home Affairs Committee over allegations that thousands of documents relating to the Stephen Lawrence murder inquiry had been shredded.

"This happened not on the watch of Bernard Hogan-Howe. It happened at least 10 years ago. It is entirely reasonable for a commissioner of the police not to know exactly when or who shredded certain documents," he said.

"I am extremely comfortable that Bernard Hogan-Howe is getting to the bottom of events that happened long before he was commissioner."

He added: "Bernard, I've got a huge respect for. I think he is doing a blinding job at the moment."