Scotland Yard has discussed with the government a radical change in race relations law to allow positive discrimination in recruitment, as the growth of London's ethnic minority populations makes the gap between the police ranks and those they serve wider than ever.

In a Guardian interview, the Metropolitan police assistant commissioner Simon Byrne said the plans the Met were examining would mean they "could only recruit, in very broad terms, a white officer if you can recruit a black or minority ethnic person at the same" time.

Byrne said current law "doesn't allow us to be as bold as we could be". Nine out of 10 Met officers are white, while the latest census data shows London's population is 40% minority ethnic.

The senior officer said the "50-50" plans amounted to "positive discrimination" which would require a change in the law. Talks were still ongoing with the government at the time of the interview, conducted the day before the Woolwich terror attack, an episode that highlighted religious and racial issues in the capital.

Some police chiefs fear overly white forces, especially in urban areas, risk damaging the legitimacy of policing as they exercise the power of the state over increasingly ethnically diverse populations.

One of Britain's most senior police chiefs also says it also makes the service less effective in fighting terrorism.

Byrne said: "We have not kept pace with the changing shape of London … The thing we have got to overcome, by legacy and history, we have broadly been, initially, a white, male-dominated organisation."

Met chiefs are interested in the model that was used to radically reform the ranks of policing in Northern Ireland, which saw one Catholic recruited for every Protestant.

Sir Peter Fahy, chief constable of Greater Manchester police, told the Guardian: "The police service is not as effective as it could be in countering terrorism because of its ethnic makeup.

"A big part of dealing with terrorism and crime is gathering intelligence, having people who get to know local people so they have the confidence to pass information."

An earlier attempt by forces to lobby the government for a law change has stalled, but sources say the Met has a stronger case as London's ethnic minority population is much higher than that of the rest of the country.

In the 2011 census, only 45% of Londoners declared their ethnicity as being white and from the United Kingdom.

The issue has dogged the Met for years, and despite improvements, the force's leaders, including Byrne and the Met commissioner, Sir Bernard Hogan-Howe, have been discussing how the Met can catch up with demographic changes.

The gap between the ethnic makeup of Met police ranks and the ethnicity of the populations they serve has never been bigger.

Positive discrimination is outlawed and would require new laws to allow measures on the mainland that were seen as successful in reforming policing in Northern Ireland and lessening its Protestant bias. The scheme lasted a decade, ending in 2011.

Fahy said efforts by the Association of Chief Police Officers to get such measures adopted nationwide had failed, as the government would not agree.

He said he now wanted to push the existing law to the limit to stipulate that any new recruits would be more likely to be chosen if they had knowledge of a language or of an ethnic community in a force's area: "Before the recession it was forecast it would take 20 years to get a representative police force … Now that is further into the distance.

"Does society and politicians accept the reality or is there to be a period of positive discrimination to change that?"

Byrne revealed the Met was also considering outsourcing its recruitment in an attempt to help shift the ethnic balance of its officers. The selection centre "tended to disadvantage people from black and minority ethnic backgrounds".

The issue is pressing for the Met as it is planning a recruitment drive for thousands of officers, after a hiring freeze. Byrne said the danger was that the force would end up missing the chance to change the ethnic makeup of recruits and "get swamped by applications from our relatives".

The National Black Police Association has said that forces are still "institutionally racist", which was the central damning finding in 1999 from the official inquiry into the Met's failings which allowed the racist murderers of Stephen Lawrence to go free for so long.

Critics of the police say that as well as the ranks being too white, police discrimination leads to minority ethnic people being more likely to be subjected to stop and search than whites, which damages confidence and trust in the police.

The rate at which black people are more likely to be stopped by officers compared to white Londoners is down from four times to twice as likely, said Byrne, and the reduction had been achieved in 18 months of reforms. The over-targeting of Asians for stops by officers, Byrne said, has now ended, according to the Met's internal figures.

Byrne said the Met commissioner had acted after hearing repeated public concerns. He said officers were now less likely to stop someone because of "where you were and the colour of your skin" and instead because of "where you were and what you were doing".

Byrne said of the stop-and-search experience experienced by black Londoners before the commissioner's reforms: "Sometimes they were probably being treated poorly," adding that better leadership of the force had helped change things on the streets. But he insisted stop and search was still a vital crime-fighting tool.