Ukip tried to persuade David Cameron to give it more seats in the House of Lords by indicating they would vote with the government "most of the time".
On Thursday 14 Conservatives, 10 Liberal Democrats, five Labour supporters and one Green were given peerages. But no new Ukip peers were appointed.
Under the terms of the coalition agreement Cameron and Nick Clegg agreed to make the House of Lords "reflective of the share of the vote secured by the political parties in the last general election".
Ukip has acknowledged it polled a "paltry" 3% in 2010, but said that would still entitle it to 22 peers. It currently has just two.
In a letter to Cameron dated 9 May 2013, six days after his party's shock local election surge that saw it capture 25% of the vote, Ukip peer Lord Pearson said the result "strengthen the case for more Ukip representation" in the Lords.
In a hand written addition to the typed letter, Lord Peason attempted to sweeten the deal for the prime minister by pledging, "and they would support the government, most of the time".
Lord Pearson wrote a second letter dated 27 March in which he attempted to persuade Cameron that granting Ukip more peers would not mean he would have to give seats in the Lords to extremist parties. "I appreciate the BNP problem, but that has faded," he said.
The Ukip peer also said he would settle for "another half a dozen" peers rather than 23 as a compromise.
However Cameron rejected the plea. The letters were published today by Ukip in frustration at the prime minister's decision.
Nigel Farage said: "It's an insult to democracy in the UK, a country that prides itself on being open and fair and the mother of Parliaments. It puts Westminster on a par with a developing world country dictatorship.
"Ukip supporters up and down the country should be enraged. The message being sent out by the prime minister is 'we don't care how you think, we don't care how you feel and not only will we wilfully ignore the way you vote we will actively block you from having any role in British politics'.
He added: "Well the more they refuse to hear our voice, the louder we will shout. Even if you are not a Ukip supporter the injustice here is apparent. They might just have shot themselves in the foot with this one."
The two current Ukip peers, Lord Pearson and Lord Willoughby de Broke, were both given their seats as Tory members before defecting to the anti-EU party.