The Pensions Board has written to the trustees of the Aer Lingus pilots pension scheme as its shareholding in the airline rises to 2.2 per cent.

The Pensions Board yesterday declined to say what it told the scheme's trustees, but it is understood it has reminded the trustees of their responsibilities to invest surplus funds wisely and with due regard to the potential return. The board, which regulates occupational pension schemes and personal retirement savings accounts, said it was continuing to monitor the situation. A spokeswoman said all pension schemes in Ireland should proceed on the basis of the "prudent person principle".

This means that those with responsibility to invest money for others should act with prudence, discretion, intelligence and regard for the safety of capital as well as income.

Yesterday the scheme, known as the Irish Airline Pilots Pensions Limited, said it now held a 2.24 stake in Aer Lingus. It bought 300,000 shares in the airline at €2.90 a share and the purchase was disclosed yesterday.

The pilots group is now one of the key players in the unfolding battle for control of Aer Lingus. Regarded - so far - as being opposed to the €1.4 billion bid by Ryanair, this could change at a later date if Ryanair improves its offer.

Other avenues for staff to increase their stake have proved fruitless so far. The Employee Share Ownership Trust (Esot) has so far failed to gather enough resources to buy additional shares, although it is still considering its options. The main pension scheme at Aer Lingus has been proposed as a potential vehicle for buying shares by some union representatives, but so far agreement has not been reached on this.

Siptu's official position remains that the Government should buy additional shares, but Fine Gael's spokeswoman on transport Olivia Mitchell last night strongly rejected this idea.

"Legally the Government must bid for the entire holding if ownership reaches 29 per cent. Siptu's proposition that having sold the company a few weeks ago for €200 million, the State should now pay €1 billion to buy it back, is unsustainable.

"The taxpayer simply could not be asked to foot a bill of that magnitude on a risky venture when there are so many other areas in needs of funds," Ms Mitchell said.