Many of the “priority investors” in Royal Mail, brought in supposedly to remain long term, stable holders of the privatised service, quietly sold millions of their shares over the summer.

The 16 investors caused controversy for the Business Secretary Vince Cable earlier this year, as many of them were precisely the kind of short-termist hedge funds the minister had pledged would not make a fast profit from the sale of the public-owned postal system.

All priority shareholders were given big stakes in the company at the flotation price of 330p a share – a valuation that came under huge criticism as the share price rocketed from the first day they began trading. Since their peak at 618p, they have come down to about 440p amid concerns about competition eating away at its lucrative urban deliveries business and a regulatory inquiry in France.

Several, like the hedge funds Third Point, Soros and Och-Ziff, sold out quickly and took multi-million pound profits. Standard Life, another priority investor, also sold its entire holding.

But The Independent’s analysis of share trading data shows priority firms JPMorgan, Schroders and Abu Dhabi’s sovereign wealth fund also slashed their holdings in recent months. The Kuwait Investment Authority, another sovereign wealth fund, sold more than £8m of shares. It remains a major investor.

JPMorgan sold £1.3m of shares last month, taking its holding down to one million shares, the Abu Dhabi sovereign wealth fund sold nearly half of its one million shares and Schroders sold more than 40 per cent of its holding for more than £2.1m – a sale likely to have profited it to the tune of more than £500,000.

Chuka Umunna, shadow Business Minister, said: “Vince Cable’s promise that the priority investors would provide long-term certainty lies in tatters. The City has cashed in at the public’s expense after being handed the lion’s share of the business, formerly owned by taxpayers.”

The biggest trade was from Artisan Partners, the US fund manager which bought in after the flotation. It ditched practically its entire stake of almost one per cent of the company last month.