Documents from early 19th century show foreign secretary helped Elgin import huge shipments of sculptures

Lord Elgin imported the Parthenon marbles into Britain without paying customs tax after the foreign secretary intervened on his behalf, newly discovered letters suggest.

The documents, dating from the early 19th century, show that after he controversially stripped them from the frieze of the Parthenon in Athens, Elgin was helped to bring huge shipments of the sculptures to Britain by the senior minister, who “knew what Elgin was up to”, according to a historian of the period.

The letters could also show Elgin deliberately underplayed the value of the spectacular sculptures, the removal of which was highly contentious even then, describing one consignment in 1803 as “trifling antiques and marbles”.

Elgin intended to display the sculptures in his stately home in Scotland, but wanted to avoid paying the high rates of customs duty levied on “sculptured marbles and curiosities”, which would have amounted to tens of thousands of pounds in today’s money.

It is now claimed he was supported by Viscount Castlereagh, the foreign secretary, who instructed officials in 1812 to allow him to import a huge consignment of 86 cases of the marbles “in the same manner as the former part of his collection, duty free”.

The historian who found the letters believes Castlereagh may have hoped eventually to acquire the sculptures for the nation, as subsequently happened in 1816 after the cash-strapped Elgin was forced to sell.

Castlereagh was the key figure persuading the British parliament to buy the marbles – itself a highly controversial act that was lampooned at the time.

“In my view Castlereagh probably always intended the marbles to become part of the national collection, and used Elgin as a convenient way of separating government from the controversial practice of taking them from the Parthenon,” said Daniel Simpson, a postdoctoral research assistant at the University of Cambridge’s Museum of Archaeology and Anthropology, who uncovered the letters among customs records at the National Archives in Kew.

If so, “this [would] implicate the government more closely in the debate about the fate of the marbles today – the government was more responsible for taking them than has been assumed”.

The sculptures have been on display at the British Museum since soon after their purchase and have remained hotly disputed ever since, with Greece continuing to demand they be handed back. The museum has proposed a “partnership” with Greece that could result in the marbles being loaned to the Acropolis Museum in Athens, although Liz Truss said this week she was not in favour of the sculptures’ return.

Dr Keith Hamilton, a former Foreign Office historian, told the Guardian that the reference in the 1803 letter – written by a civil servant to customs officers based on Elgin’s submissions – to “some trifling … marbles” was “certainly a considerable understatement” which showed Elgin was trying to exploit his status as a diplomat to avoid tax.

The 1812 letter “does … indicate that Castlereagh knew what Elgin was up to”, said Hamilton. “He was well aware of the quality and quantity of Elgin’s ‘purchases’, and he may very well have had prior knowledge of the former ambassador’s intentions.”

However, Hamilton said the letter did not offer cast-iron proof of British government involvement of “what some would see as plunder”. Elgin was no longer a diplomat then and it is possible the intervention by Castlereagh was “the sort of thing that in 1812 and later one gentleman did for another”, he said.

Geoffrey Robertson KC, a leading barrister and advocate for the marbles’ return, said the new letters “add to the considerable evidence that the British government was responsible, in law, for Elgin’s heist, an unauthorised removal he achieved by copious bribery of local Turkish officials so they would turn a blind eye while his workmen ripped the statues from the temple walls. In international law, the conduct of an ambassador engages the responsibility of his state.

“These new documents show that the government aided and abetted the theft by permitting the marbles to enter without customs inspection or duties.”

In a statement, the British Museum said it “follows all new research into the history of its collection with interest. We are open and transparent about the heritage of objects in our permanent collection. Establishing the provenance of an object has been an integral part of the museum’s acquisition process for decades.”

A spokesperson for the UK Department for Digital, Culture, Media and Sport said: “The Parthenon sculptures in the British Museum are legally owned by the trustees of the museum, which is operationally independent of government. Decisions relating to the care and management of its collections are a matter for the trustees.”