The first Liberal Democrat group openly opposed to the coalition is to be launched at the party's spring conference in Gateshead next month with a warning that the coalition has been a political disaster for the party, as well as a denial of its radical roots.

Launching a website on Wednesday, the group Liberal Left said it hoped to become a rallying point for members opposed to the coalition and those who see the party as a centre-left organisation seeking common cause with Labour, Greens and others on the centre left.

One of its founders, Richard Grayson, conceded that the vast majority of the party was committed to the coalition and denied the group would be working to put a motion to conference calling for the Liberal Democrats to withdraw from its partnership with the Conservatives. He said the focus was more on developing policies on the centre left, and creating a space for a coalition with Labour if necessary after a general election.

Most of the group's founder members have long been opposed to the coalition, but it believes other party members will join, and the mood of the party is radicalising. Grayson said Liberal Left differed from the other well-established left group inside the party – the Social Liberal Forum – in that it opposed the coalition, and did not agree that the party should be politically equidistant between Conservatives and Labour.

In its strongly worded founding statement, Liberal Left asserts: "We articulate policy positions within the Liberal Democrats which should be central to a radical party. Such policies have informed recent general election manifestos which our candidates have stood, and on which our MPs have been elected.

"Those views are not being currently voiced effectively in a party whose radical traditions have become muted in government and whose leaders have taken the party's policy positions to the right. We are now part of a government which is Eurosceptic, neo-liberal and socially conservative."

It also calls for a different economic strategy, one that is "not based on demonising the poor nor apparent overspending by a previous government (spending which Liberal Democrats did not say should be reduced)".

It says: "The popularity of progressive single issue campaigns shows a genuine appetite for progressive politics. We believe the Liberal Democrats should be part of this politics, not its target."

It also claims the coalition has been "politically disastrous leading to a haemorrhage of support, activists members and councillors". It claims policy gains such as tax allowances for the poor and the pupil premium for poorer children have been dwarfed by losses such as the VAT rise and loss of standards and funds in education.

It claims that if coalitions are to become more frequent, voters should not be left in the dark over the party the Liberal Democrats would partner with. It argues: "Many of the political problems faced by the current coalition flow from it being a government which most Liberal Democrat voters did not want. It is ideologically unsustainable and without a mandate.

"A future coalition with Labour and others on the liberal left is more likely to secure Liberal Democrat goals than a further coalition with the Conservatives and we should actively work to make that possible."

Asked to pinpoint the three strongest policy differences that Liberal Left had with the coalition, he said the deficit, tuition fees and the role of city academies in education. Grayson acknowledged that at a special conference immediately after the election the party voted overwhelmingly for joining the coalition, but he said there was a long tradition of dissent inside the Liberal Democrats. "We have never been a democratic centralist party in which the whole party has to abide by a conference decision for ever more."

The group's launch was greeted with derision by some Liberal Democrats, claiming it was a campaign by people who should be in the Labour party. Grayson said he hoped his group would have good relations with the Social Liberal Forum, but said: "They want to work incrementally and often in private to influence policy. That is a legitimate approach, but we have a different view."