The Bow Group said it was "imperative" figures such as former Tories Douglas Carswell and Mark Reckless - who defected and then won back their Commons seats as Ukip MPs last year - were returned.
Accusing the Conservative leadership of "sleepwalking" to a Labour win by failing to acknowledge an overall majority was out of sight, it bemoaned the lack of a formal "accommodation" with the smaller party and said the tactical switch should also happen across much of the north of England.
Founded in 1951, the Bow Group lists Lords Howe, Howard, Heseltine and Lamont among its patrons. Ex-prime minister Sir John Major was president until he stepped down earlier this year.
"In northern seats, as well as in key Ukip targets such as Rochester and Strood, the Bow Group, which was Margaret Thatcher's think tank of choice, has reflected on the situation across the country and has come to the conclusion that given Ukip's strong positions on defence, taxation, an EU referendum and uncontrolled immigration, that they are best-placed to counter the Labour threat in areas such as Heywood and Middleton, Great Grimsby, Boston and Skegness, Barrow in Furness and all across the north of England," it said in a statement.
Chairman Benjamin Harris-Quinney, a consistent critic of David Cameron's leadership, told The Telegraph: "Of course our preference at this election is a Conservative majority, but few in the Conservative Party will acknowledge the reality that this is now very unlikely to happen, and without that acknowledgement we are sleep-walking into a Labour government.
"We recognise the need to keep Ed Miliband out of Number 10 and the best way to do this is for Conservative voters to lend their votes to Ukip, who are best-placed to beat Labour in many areas.
"It is also imperative that leading conservative voices like Nigel Farage in South Thanet, and incumbents Douglas Carswell and Mark Reckless, are returned to Parliament in order to balance the far left-wing rhetoric of minor parties like the SNP, Plaid Cymru, the Green Party and the Liberal Democrats in the House of Commons chamber.
"It is a strategic failing that no accommodation could be made with Ukip prior to the election, but in acknowledging there won't be a Conservative majority we have to now be realistic that the best chances of forming a Conservative government lie in alliances between parties of similar values like the Conservatives, Ukip and the DUP."