Sotheby's and Christie's are presenting musketeer paintings by Picasso as their most highly valued lots of the summer season.

A showdown between two musketeers is to be the art market's main attraction as Sotheby's and Christie's shape up to beat not only each other, but also the economic recession at London's Impressionist and Modern Art sales, which go on view next week.

The musketeers in question are two paintings by Pablo Picasso made in the twilight of his life. In 1965, at the age of 84, Picasso was admitted to hospital for surgery on an ulcer. While recuperating, he read Alexander Dumas's classic historical romance, The Three Musketeers. Facing sexual impotence and the prospect of death, the novel inspired him to paint with new bravado and intensity. Made rapidly with vigorous, gestural brushstrokes and almost childlike simplicity, they are autobiographical images of a man fighting against old age, and winning.

When they were first exhibited at the Palais des Papes in Avignon in 1970, the musketeer paintings met with critical hostility. Picasso had defended his later style saying: "When I was a child, I could draw like Raphael, but it took me a lifetime to draw like a child." But his dealer, Daniel Henry Kahnweiler, rejected the new works, and his former friend and apologist, the art historian Douglas Cooper, denounced them as "incoherent scrawls done by a frantic old man in death's antechamber".

It was not until the late Eighties that interest in them was rekindled by the young, fashionable neo-expressionist artists of the time, Georg Baselitz and Julian Schnabel, who were at the forefront of a renewal of interest in painting. Among the prescient collectors who began buying them was the financier Gilbert de Botton, whose company sponsored the Tate Gallery's exhibition of late Picasso in 1988.

But it has only been recently that late Picassos have been making serious money, boosted by the idea that they were cheaper than his older works and more contemporary-looking. Last May in New York, a late painting belonging to Schnabel sold for $7.7 million

(£5.2 million), but the star of the sales was Mousquetaire à la pipe, a musketeer painting from the collection of Madoff victim Jerome Fisher, which sold for $14.6 million dollars (£9.8 million) – a record for a musketeer painting at auction. It is a measure of how much prices had increased that Fisher had bought the painting in 2004 for $7.2 million.

Two of the most talked about exhibitions of this year have been the National Gallery's Picasso: Challenging the Past, in which it was shown how the late works made reference to the Old Masters, and Mosqueteros, an exhibition at the Gagosian gallery in New York. At this exhibition most works had been lent by museums and members of Picasso's family, but a few were for sale, with prices up to £12.5 million.

It is against this backdrop that both Sotheby's and Christie's are presenting musketeer paintings by Picasso as their most highly valued lots of the summer season. At Christie's is a 5ft high canvas, Homme à l'épée, painted on July 26 1969. A confrontational figure brandishing a sword, it was one of the largest in the Avignon exhibition and is estimated at £5 million-£7 million. Four years ago it was bought by a London-based collector for £2.7 million, and it is now being sold by his family.

Coincidentally, Sotheby's musketeer, also called Homme à l'épée, was painted the previous day. Only a few inches smaller, it has the advantage of having never been at auction before, and the distinction of having graced the cover of the Avignon exhibition catalogue. It carries the slightly higher estimate of £6 million-£8 million – a price at which Sotheby's has declared it has an "irrevocable bid", meaning that someone has agreed to bid to that level even if there is no one else bidding on the picture.

Now that neither saleroom offers guarantees because they lost so much money on them last autumn, the "irrevocable bid" is Sotheby's way of securing high-value material without the risk of having to make a guarantee themselves.