Raphael's Madonna of the Pinks could now be saved for the nation after a lottery fund stepped in to give the National Gallery £11.5m to help it bid for the art work. But how are works of art "saved" in this way? BBC News Online outlines the obstacles in the way of exporting important works of art.
Obstacle one: The export licence
Culture Secretary Tessa Jowell is in charge of appointing experts to the committee
Any work of art which is more than 50 years old, and valued above certain amounts needs a licence to be exported to another country. The values concerned depend on what kind of object they are.
This applies even if it is just being loaned out display at a foreign gallery. As well as works of art, other "cultural goods" including books, archaeological finds, and industrial objects are included in this scheme. These are dealt with by the export licensing unit at the government's Department for Culture, Media and Sport (DCMS).
Once the unit gets the licence application, an export adviser can object to it if it is deemed to be of national importance - known as the Waverley criteria, after a chairman of a 1950s export committee.
Between 25 and 50 objections are made each year, out of between 8,000 and 10,000 applications.
Obstacle two: The committee
The Waverley criteria are:
Is it so closely connected with our history and national life that its departure would be a misfortune?
Is it of outstanding aesthetic importance?
Is it of outstanding significance for the study of some particular branch of art, learning or history?
The refused licence application then goes to the Reviewing Committee of the Export of Works of Art. The eight members of this panel are appointed by the Culture Secretary, and each of them has a particular field of expertise.
They will then hold a meeting with the committee, the adviser who recommended blocking the sale, the seller, plus DCMS officials and independent advisers.
The committee will then decide to either grant the export licence, or defer it to enable a new offer to be made by a buyer who can make the work available to the public.
This offer can be made at a fair market price - although a final decision on this will be made by ministers and their advisers at a later stage.
Obstacle three: The invitation
The arts minister - currently Estelle Morris - will make a final decision on the matter, and will set the length of time the licence will be deferred for.
Then potential bidders are invited to make an offer to the committee for the object.
If nobody bids, an export licence will normally be granted. But if reasonable bids are made, the government has ways of persuading sellers to accept them - or drop their plans to sell.
Obstacle four: The ministers step in
If a public body makes an offer, the seller is free to accept or reject it. But if the bid is rejected, the arts minister is likely to refuse the export licence - making it almost impossible to take the work outside of the UK.
If a private body bids for it, the same conditions apply if the bidder agrees to care for the object and allow the public access to it.
Estelle Morris was appointed to her job last month
Obstacle five: Tax
In the case of Raphael's Madonna of the Pinks, the National Gallery decided to mount an appeal to buy the painting - for which the Getty Museum has already put in a reported bid of £29m.
The gallery has been advised a fair market price would be £21m - because the Duke of Northumberland does not have to pay tax on the sale to the Gallery.
But he would have to pay take on a private sale.
These tax rules are in place to encourage owners of artworks to sell them to UK institutions rather than send them abroad.
It has opted to fund this with £11.5m from the Heritage Lottery Fund, and £9.5m from its public appeal.
So if the gallery can raise the £9.5m it needs to top up its grant, the painting is likely to stay in the UK.
But, if a bid is put in, a final decision will not be made until 27 August, when Estelle Morris and her advisers have completed their deliberations, and decided whether the National Gallery's offer consitutes a fair price.