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.

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.

Had Ukip voters given their support to the Tory candidate in all these seats, Mr Cameron would now enjoy a wafer-thin overall majority in the House of Commons, and would not be forced to see a deal with the Lib Dems.

After being released from the John Radcliffe Hospital, Oxford, where he was treated for injuries suffered in a polling day plane crash, Mr Farage told the Press Association: "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."

Looking back on his own unsuccessful campaign to oust Speaker John Bercow in Buckingham, Mr Farage added: "I am glad I did it, and I am glad to be alive.

"You can't backtrack in life. I made a decision in September, and now here we are. I've got more time on my hands now to reflect on that than I planned to."

He said Mr Bercow was "obviously liked" as a local MP, but the fact there were 10 other candidates standing meant the anti-Speaker vote was "very, very split, which made it difficult".