David Cameron is at the centre of a cash-for-peerages row following the appointment of two wealthy Tory backers to the House of Lords.

Financier and party co-treasurer Michael Farmer and businessman Ranbir Suri are among 12 new Conservative working peers announced today by the Prime Minister.

According to an analysis by the Electoral Reform Society (ERS), which campaigns for an elected upper chamber, Mr Farmer has donated more than £6.5 million to the Conservatives.

Mr Singh, a former general secretary of the Board of British Sikhs, has contributed a further £312,435 - either personally or through his company Oceanic Jewellers, the ERS said.

ERS chief executive Katie Ghose said such appointments meant wealthy political donors were effectively able to buy a place in the legislature.

"These appointments further cement the impression that to get into the House of Lords, all you have to do is write a fat cheque to a political party or be a party hack," she said.

"The second chamber is a crucial part of our political system, with real legislative power. It cannot be right that people are effectively able to buy a seat at the highest level of politics."

The Tories strongly defended the appointment of Mr Farmer, describing him as a "worthy recipient" of a peerage.

"He has been involved in numerous charities and is co-founding sponsor of a successful academy chain school in south London," a Tory source said.

The party pointed out that in the past, wealthy Labour donors had been made peers including Lord Sugar and Lord Sainsbury.

The ERS said that of the 21 new working peers appointed by the three main political parties, six had made donations to them - either directly to the party or to individual candidates - while 16 had held some form of previous political position.

It said that of the new Liberal Democat peers, Barbara Janke, former leader of Bristol City Council, has donated £5,498, while ex-Sheffield City Council leader Paul Scriven has given £2,000.

On the Labour side, ex-East Enders actor Michael Cashman has donated £2,500 while the publisher and chair of Penguin Random House, Dame Gail Rebuck, has given £2,000.

The ERS pointed out that Dame Gail's late husband, Labour peer Lord Gould, had also previously made donations totalling £31,250.

In other appointments, businesswoman and West Ham vice-chairman Karren Brady and former Marks and Spencer store chief Sir Stuart Rose have been nominated as Conservative peers.

They are joined by Andrew Cooper, Mr Cameron's former director of strategy who left No 10 after reportedly falling out with other members of his inner circle, and former Tory leader in the European Parliament Martin Callanan.

TalkTalk chief executive Dido Harding, Mr Cameron's digital adviser Joanna Shields, and director of the New Schools Network charity Natalie Evans also join the Conservative ranks in the Lords.

The Tory list is completed by Arminka Helic, the Government's leading adviser on preventing sexual violence in conflict, Nosheena Mobarik, a former chairman of CBI Scotland and chairman of the Pakistan Britain Trade and Investment Forum, and Carlyn Chisholm, a senior volunteer in the party.

For the Liberal Democrats, Nick Clegg has appointed the party's former chief executive Chris Fox, and former council leaders David Goddard from Stockport and Kath Pinnock from Kirklees. His list is completed by Cambridge academic and city councillor Julie Smith.

Labour leader Ed Miliband has appointed the party's former deputy secretary general Chris Lennie while the Democratic Unionists have nominated the Speaker of the Northern Ireland Assembly, William Hay, although he will sit in the Lords as an independent crossbencher.

Mr Farmer said he was " greatly honoured" to have been nominated by the Prime Minister to become a Conservative peer.

He said he was looking forward "to working and supporting the Government in the House of Lords, in particular in its important long-term work for the economy, and in its ongoing reform of our welfare and educational systems".