Greens today urged Liberal Democrats to defect to them following what they described as Nick Clegg's "betrayal" of members and voters by taking the party into coalition with the Tories.

Making their own "big, open and comprehensive offer" to Lib Dems, the Greens said the coalition deal left them as the only party campaigning for "real democratic change, real action to tackle poverty, and real action on climate change".

The party's first MP, Caroline Lucas, is taking up her seat in the House of Commons today following her victory in Brighton Pavilion at last Thursday's General Election.

Ms Lucas, who is leader of the Green Party of England and Wales, said: "The Lib Dems have shown themselves to be not so much a party of change as a party of changing its mind.

"The Lib Dems have made themselves known as a party of dirty tricks in election campaigns. But now Nick Clegg has carried out the biggest Lib Dem dirty trick so far, betraying all those people who voted Lib Dem because they honestly thought it would bring about electoral reform."

The call to defect was made jointly with the Scottish Green Party, whose MSP Patrick Harvie described the Lib Dems' decision to go into coalition as "extraordinary".

"Thousands of their activists and voters will feel heavily betrayed today," said Mr Harvie.

"Many explicitly campaigned as the best way to keep the Tories out of power, as a party of radical change and a party of principle, and they have now been completely let down by Nick Clegg and his top team. These members and supporters did not work hard over the last weeks and months to see their party become a wholly-owned subsidiary of the Tories.

"We would therefore like to make a big, open and comprehensive offer to Lib Dem supporters to come now and talk to the Greens instead. Many former Lib Dem members have already found a long-term home with the Greens, including former Lib Dem councillors.

"Thousands of Lib Dems will be unable to stomach this decision to put David Cameron's Tories into power, and it's time for them to consider coming home to the Greens."