Artist John Constable stepped back to admire his painting of Brighton's Chain Pier and then took a knife and vandalised it.

This is the latest theory about a mystery which has captivated the art world for decades.

The painting of the city's bustling seafront was a tour de force which took the artist months of careful refining.

So art historians were baffled why anyone would want to cut nine inches out of the canvas - removing a boat and several figures from the right-hand side.

Now it is believed the artist himself made the sweeping alterations in an effort to make more money, believing a smaller piece would be more valuable. However, his plan backfired as the piece remained unsold during his lifetime.

Constable's original sketches were full-size and reproduced faithfully. They made it clear to art historians that sizeable slices of the finished work had been removed.

The canvas, painted in 1827, is one of several of Constable's "six footers" which will be exhibited at London's Tate Britain from tomorrow.

Until a recent forensic analysis, it had been assumed the oil painting had been altered some time after Constable's death in 1837 - ruling out the artist as the culprit.

But Sarah Cove, the founder of the Constable Research Project who carried out the analysis, said: "It was almost certainly done by him.

"As the oil paint dries it becomes very brittle over time. If the painting is cut and re-stretched, it cracks in a certain way. But what we could see was more of a stretched look to the paint at the edge, which is what we get when it is more plastic and not completely hardened.

"It suggests it was cut not long after it was painted."

Constable made frequent visits to Brighton from 1824 with his sick wife Maria, producing numerous drawings.

But the Chain Pier, Brighton, was his only large-scale painting of the area. The pier, which opened in 1823, was destroyed by a storm in 1896.

Ms Cove said the drastic alteration in size was likely to have made it more manageable for any potential buyer.

She said: "His clients weren't aristocrats living in stately homes. It wasn't the sort of thing you could put on the average wall of a Georgian house. But he didn't really have much luck with selling his work."