As Prime Minister David Cameron was due to join leaders of Libya's Transitional National Council (TNC) at a meeting in Paris this evening, Mr Hague said help getting justice for the officer was key.
He told BBC Breakfast: "It is an important priority. There are, of course, several important priorities. The whole stable future of Libya is a very important priority.
"But we have always made clear to the Transitional National Council, including when their leaders came here in May, that this was a very important matter for the United Kingdom."
He added that the Libyan representatives, including TNC leader Mustafa Jalil, had promised to co-operate fully.
But he added that Britain was not setting a timescale as the Metropolitan Police are investigating the 1984 murder in which WPc Fletcher was shot outside the Libyan embassy in London.
Only one of three main suspects is believed to be still alive but Mr Cameron has said he is sure the TNC will assist a British police investigation.
The Prime Minister will jointly host this evening's conference with French president Nicolas Sarkozy to build international support for the fledgling rebel administration.
The international conference - aimed at helping establish a free and democratic Libya - will also allow the TNC to set out what humanitarian and other aid it needs as it seeks to rebuild the country after 42 years under Colonel Muammar Gaddafi's rule.
US secretary of state Hillary Clinton and United Nations secretary general Ban Ki-moon will also attend the high-level "Friends of Libya" meeting.
Russia and China - UN Security Council permanent members which did not back its resolution which allowed the military action to protect civilians from Gaddafi's regime - will be sending representatives to the conference.
Diplomatic efforts to unlock Libyan assets will be stepped up to provide funds for the TNC, with the possibility of a new UN resolution on the frozen resources set to be discussed.
The first batch of almost £1 billion of Libyan dinar banknotes, which were seized after being printed in the UK, have already been sent to Libya after the UN sanctions committee agreed to a request from the British Government.
Around £140 million of newly-minted Libyan banknotes were transported to the Central Bank of Libya in Benghazi by the RAF.
They were part of a stock of 1.86 billion dinars (£950 million) printed in the UK, which were frozen under United Nations sanctions at the start of the crisis in Libya.
The Foreign Secretary said he was "delighted" that the RAF had delivered the money.
He said: "The banknotes will be used to pay the wages of Libyan public sector employees, including nurses, doctors, teachers and police officers; provide support for those on social security who have not been paid for a number of months; provide aid for refugees displaced by the conflict; pay for medicines and subsidies for food supplies; and will be transferred to commercial banks in order to make currency available to the general public for their basic living expenses.
"This is particularly important at the time of Eid.
"Returning money to the Libyan people is part of our commitment to help the TNC rebuild Libya and help create a country where the legitimate needs and aspirations of the Libyan people can be met.
"It follows and was authorised by the UN's decision to unfreeze 1.86 billion Libyan dinar banknotes printed in the UK. Further deliveries of the remaining funds will be made shortly."
Defence Secretary Liam Fox said: "I am pleased that the RAF was able to assist in the delivery of these banknotes - another example of the highly professional approach all three of our armed forces have been taking to support the Libyan people at this time."
In Libya, two of the embattled dictator's sons gave conflicting messages about the regime's plans for the future.
Saif al-Islam vowed to carry on fighting in an audio message broadcast on Syria's Al-Rai television.
Claiming he was speaking from the suburbs of Tripoli, he said his father was safe.
"We are going to die in our land," he said. "No one is going to surrender."
But his brother al-Saadi Gaddafi claimed to be authorised to negotiate with the TNC in an effort to end the bloodshed.
The rebel commander in Tripoli, Abdel Hakim Belhaj, said al-Saadi rang him to discuss giving himself up.
Sir Menzies Campbell, a member of the Commons Foreign Affairs Select Committee, said it was a sign that the despot's regime was crumbling and called on Col Gaddafi to "accept the inevitable" and surrender.
Mr Belhaj said al-Saadi first called him on Tuesday and asked whether his safety could be guaranteed.
He said: "We told him 'Don't fear for your life. We will guarantee your rights as a human being and will deal with you humanely'."
The claims have not been confirmed but former Liberal Democrat leader Sir Menzies said: "If these reports are true, then the wheels are really coming off the Gaddafi camp.
"A great deal of bloodshed could be avoided if Col Gaddafi himself would accept the inevitable."
The Times reported that Mr Cameron commissioned a secret team during the Libyan crisis, the Libyan Oil Cell, which played a part in helping the rebels.
The newspaper said it was created in April and based in the Foreign Office after the first flush of bombing failed to oust Gaddafi.
After it was realised that hastily-arranged sanctions were hurting the rebels while leaving Gaddafi's war machine untouched, it pursued policies such as aiding the blockade of Gaddafi-held ports, the report claimed.
Ed Miliband said it was important for the Paris meeting to establish that the Libyan people will "determine their own future".
The Labour leader said: "This is an important moment first of all to recognise the NTC and the role they will be playing in taking Libya forward.
"We have got to be led by them because it's very important that the Libyan people determine their own future.
"What we need is order restored on the streets of Libya. We also need to provide assistance to that National Transitional Council."
In a message broadcast on Syrian TV, Col Gaddafi remained defiant and warned the rebels they faced a "long battle".
He said: "Let it be a long battle. We will fight from place to place, from city to city, from mountain to mountain.
"Let it be a long battle so that we can show to them that they cannot rule the Libyan people, they cannot subjugate our tribes."
He added that his forces would fight "in every street, every village and every city".