David Cameron's refusal to accept an offer of electoral co-operation from the UK Independence Party may have cost him the chance of leading a Conservative majority government, Nigel Farage said last night.

Ukip said last year that its candidates were ready to stand aside in seats where they could harm Tories in return for a referendum on Britain leaving Europe.

Conservative activists have compiled a list of 21 seats where the votes secured by Ukip candidates outstripped the margin between a second-placed Tory and the Labour or Liberal Democrat victor.

Mr Cameron is currently negotiating a deal with the Liberal Democrats in a bid to form the next Government.

After being released from Oxford's John Radcliffe Hospital yesterday, where he was treated for injuries suffered in a polling day plane crash, Mr Farage said: "In parts of the South and West, it's quite possible Ukip has led to a hung Parliament, in which case Mr Cameron should have accepted my offer last year.

"We said that if he promised to give us a referendum, we wouldn't field any candidates, but he refused."