The former Labour Prime Minister Gordon Brown has called for 100,000 Scottish people to sign a petition demanding Westminster honours its vows to devolve further powers to the Scottish Parliament.

In a letter to his constituency party, Brown called for £4bn of VAT to be transferred to Holyrood, as part of a 14 point plan for Scotland's political power.

Also included as part of his plan are increased powers over investment, job creation, transport, welfare, elections, taxation and the majority of income tax control.

Brown was an instrumental figure in the 'No' campaign's efforts to defeat a resurgent 'Yes' vote on 18 September's Scottish referendum over independence, outlining a timetable for further devolution and enlisting the support of the three main political parties in London for his plan.

However, since the vote, there have been fears that the coalition government will not fully honour its pledge to devolve further powers to Scotland, with the debate being marginalised by the discussions over devolution for England.

Brown said: "The vow made by three party leaders on September 16 was a self-standing set of promises to the people of Scotland.

"Yet immediately after the referendum result, a new proposal that was never raised in the pre-referendum discussions and yet being material to the referendum should have been raised before the vote was introduced - to lower the status of Scottish MPs in the UK when voting on matters including tax.

He said that the pre-referendum agreement signed by the pro-devolution parties "contained no ifs, no buts and had no conditional clauses and no strings attached", adding that "the Tory trap that we are in danger of falling into is to devolve all decisions on Scotland's income tax rates away from Westminster and then to deny Scotland representation in votes on budget decisions on income tax rates".

He warned that further delays would further imperil the union, just weeks after unionists claimed such a historic electoral victory.

"They must now demonstrate that it is not true that on the morning after the referendum the Conservative Party stopped thinking about Scotland and started thinking only about the Conservative Party," Brown said.

Scottish nationalists have leaped on Brown's comments, showing that they prove unionists signed up for actions they must deliver immediately and that the letter proves that the further devolution was not, as intimated pre-referendum, a done deal.

Alex Salmond, the outgoing Scottish First Minister, said: "The 'Tory trap' is not the proposals on income tax which Gordon Brown talks about – it is the Tory trap which he and his colleagues are leading people into, in which the issue of more powers for Scotland becomes entangled in a row between factions of the Westminster establishment.

"Scotland has been promised very substantial new powers, regardless of Tory attempts to link the issue to that of English votes for English laws."