Heidelberg shares jumped £2.56 (€3.78) to £27.48 on Monday, before closing on £26.50. Some reports have suggested that rumours the firm could be the target for a buy-out are behind the jump.

However, City analysts have suggested that the share price spike was due to a surprise decision, announced on Monday, by Heidelberg’s former majority shareholder RWE, the energy and water group, to hold on to shares it owns in the company.

It had been expected to distribute the shares to investors in a £312m bond, which is due to mature this month. Instead, it will give cash to the bondholders and retain the shares, which represent a 15% holding in Heidelberg.

Heidelberg spokesman Thomas Fichtl said: “Some parties in the financial market were surprised and may have had to buy other shares to keep their promises.”

RWE issued the bond after selling a 35% stake in Heidel­berg direct to the stock market in 2004, saying it did not fit with its core businesses.

Fichtl said: “What RWE wants to do with the shares – this we do not know.”