James Watt, the Scottish genius whose experiments led to the evolution of the modern steam engine, could actually have hampered the country's industrial development, it was claimed yesterday.

If Greenock-born Watt had been less cautious his inventions could have paved the way for the development of the steam locomotive some 15 years earlier, according to a (pounds) 700,000 archive of correspondence which has surfaced at auction for the first time.

Watt's development and patent of a separate condenser for the steam engine in 1769, increasing threefold the efficiency of the old atmospheric engines, was possibly the single most important invention of the eighteenth century.

It has been claimed that the 25-year patent restricted anyone making adjustments to it and caused the progress of the world's first steam locomotive to be put on hold.

''If Watt had been brave enough to forge ahead then Britain would not have had to wait until 1825 for the opening of the first public railway,'' David Park, manuscripts specialist for auctioneers Bonhams said yesterday.

More than 1100 letters from Watt and Matthew Boulton, his partner in the Soho Manufactury in Birmingham, to Thomas Wilson, their agent in Cornwall, reveal that the patent protected the invention and made them money but also restricted progress and innovation, according to Mr Park.

''The trouble was Watt did not think man was capable of handling high pressure steam. Basically, he was afraid of it.''

''As it was, a younger generation of pioneers was prevented by Watt's patents from exploring the possibilities of high-pressure steam.''

James Gibson Watt, the inventor's great-great-great-great grandson, said such allegations were ''spurious''.

''He had justifiable safety concerns about copies of his invention and did what any sensible businessman would do to protect it from plagiarism,'' he said last night.

''This secured the future of the Soho works which employed many people and allowed him to refine his invention, which was to everyone's advantage.''