London, May 30 (Reuters): Spanish construction firm Grupo Ferrovial raised its bid for British airports operator BAA Plc to ?9.73 billion ($18.1 billion) on Tuesday but BAA said it was still too low.

Ferrovial, which raised its offer to 900 pence-a-share in cash for the owner of London’s Heathrow, Gatwick and Stansted airports from the previous proposal of 810p, said BAA’s claim last week to be worth more than 940p a share was “not credible.”

It said the new bid, which also includes offers for BAA’s convertible bonds, was 41 per cent above BAA’s average share price for the 30 days ended February 6, the day before speculation emerged about a possible offer for the British group.

“The board of the offeror believes that the increased offers represent extremely attractive value for BAA shareholders and BAA convertible bondholders respectively, and hence is disappointed that BAA has chosen to reject such offers without further discussion,” Ferrovial said in a statement.

The revised offer sent BAA shares up 6.6 per cent to 874-1/2p by 0840 GMT, the biggest gainer in the London market’s FTSE 100 index. It is still significantly below the 900p offer price.