The museums have all carefully weighed the risks involved

National Gallery

Collections in Berlin, Los Angeles, Naples and Pisa are all contributing elements of the altarpiece, which originally consisted of 19 joined panels and was last seen intact in 1568.

The show is due to open to the public on 12 September.

But the exhibition has sparked attacks on the National Gallery from two sides.

Reproductions

James Beck, founder of the New York-based ArtWatch International organization, accused the gallery of pressurising the smaller museums into loaning the works - and argued that the show lacks scholarly merit and popular appeal.

Mr Beck has urged the four museums involved in sending panels to London, which include Italy's Capodimonte in Naples and the National Museum of San Matteo in Pisa - to send reproductions instead of the originals.

The painter was an early master of perspective

The surviving works, Mr Beck argues, are extremely delicate and prone to cracking.

The National Gallery has now accused Mr Beck of defamation and asked that he clarify what the gallery called his "misleading" statements.

In a letter to Mr Beck the gallery's director, Neil MacGregor, said: "Every museum weighs carefully the risks involved in transporting a picture, and I am sure you would not suggest that any of those involved in this venture are unmindful of their responsibilities."

Another attack on the exhibition has come from the Italian undersecretary of culture, Vittorio Sgarbi, in an interview in Italian newspaper La Repubblica.

Loans

The minister said that the National Gallery had failed to match Italian generosity in lending artworks.

And he said that he would try to stop the loans arranged for the Masaccio exhibition - which the National Gallery has said took two years to negotiate and will reunite the pieces with a central panel The Virgin and Child, held in London.

The minister singled out the National Gallery's refusal to lend paintings to a show in San Severino Marche in Italy - a show which Sgarbe had himself curated.

"To the National Gallery I say: We accept your method and we apply it," the minister was quoted as saying.

'Risks'

In a statement the National Gallery defended its intentions to reunite the elements of Masaccio's altarpiece: "The London panel is too large to be able to travel safely, but the other surviving fragments are all much smaller.

"The museums in which they are held have all carefully weighed the risks involved and have come to the conclusion that, with modern systems of packing, transport and climate control, they can responsibly lend them to London," it said.

A spokeswoman told BBC News Online that the National Gallery had been assured by the Italian galleries involved that their paintings would be part of the exhibition.

Tommaso di ser Giovanni, known as Masaccio, was born on 21 December 1401 in San Giovanni Valdarno near Florence.

He is regarded as a master of perspective and one of the pioneers of the Italian Renaissance.